Title:
MULTIMODE SELF-CHECKING FLAME DETECTOR
United States Patent 3683372


Abstract:
An alarm signal is operated by a multimode self-checking circuit upon failure of such circuit, or failure of a flame being monitored by a device that is sensitive to such flame. This circuit is provided with a plurality of different drive circuits each of which is designated for a flame sensitive device, such that the flame can be monitored.



Inventors:
HORN ROBERT
Application Number:
05/147565
Publication Date:
08/08/1972
Filing Date:
05/27/1971
Assignee:
ROBERT HORN
Primary Class:
Other Classes:
340/521, 340/529, 340/578, 431/24
International Classes:
G08B17/12; (IPC1-7): G08B29/00
Field of Search:
340/410,227 431
View Patent Images:
US Patent References:



Primary Examiner:
John, Caldwell W.
Assistant Examiner:
Robert, Mooney J.
Attorney, Agent or Firm:
John Maier III, Et Al
Parent Case Data:


CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part of application Ser. No. 849,401, filed on Aug. 12, 1969 and now abandoned.
Claims:
1. A multimode flame detector system capable of use with any selected one of a plurality of different types of ultraviolet and photosensitive tubes that are sensitive to rays emitted by a flame to be monitored thereby comprising means for indicating when such flame is in an "out" condition, a control circuit containing an amplifier circuit for operating the "out" condition indicating means, a plurality of individual drive circuits corresponding to such tube types, each being capable of delivering flame condition signals suitable for reception by said amplifier circuit when energized and connected thereto; means for connecting the selected type of tube exclusively to the drive circuit corresponding thereto, and means for delivering only the output of the so connected drive circuit to the input of said amplifier of the control circuit; whereby the system functions equally well with any selected type of ultraviolet and photosensitive tube that is suitable for monitoring such

2. A multimode flame detector system as defined by claim 1, in which a unitary circuit module contains said drive circuits and tube connection

3. A multimode flame detector system as defined by claim 2, in which said module consists of a circuit board containing two different UV tube drive circuits and a phototube drive circuit, a wave shaping circuit for said UV tube drive circuits, means for connecting a selected one of said UV tube drive circuits to said wave shaping circuit, and means for connecting the output of either said wave shaping circuit, or

4. A multimode flame detector system as defined by claim 3, in which said control circuit includes self checking circuit means comprising means acting to periodically blind the tube to such flame when periodically energized to check the output of the circuit for the so simulated flame loss, means acting to remove such blind when the circuit is functioning normally, and means acting to operate said means to indicate a circuit malfunction when

5. A system as defined by claim 4, including means for adjusting the sensitivity of the amplifier circuit to the response of the tube and, hence, the flame condition, and means for adjusting the amplifier for varying background flame conditions

6. A system as defined by claim 5, including means for preselecting such sensitivity adjustment for at least two settings, and

7. A system as defined by claim 5, including

8. A system as defined by claim 5, including independent "Flame On" and

9. A system as defined by claim 5, in which solid state components

10. A system as defined by claim 5 in which said checking circuit is a component of the amplifier-control circuit.

Description:
It has been proposed in the past to use glow discharge flame sensing tubes which "see" the flame and receive their firing energy therefrom.

Radiation detector tubes of the Geiger-Muller type, or ultraviolet (UV) type, requiring a particular drive circuit for each type, have also been proposed. A photosensitive flame monitoring circuit has also been proposed.

None of the prior systems is entirely satisfactory in actual use, and each type of tube sensitive to the flame being monitored required its own particular drive circuit. For example, ultraviolet tubes of one type required a particular circuit; those of another type of different circuit; and photosensitive type tubes still another different circuit. Prior systems were not interchangeable, insofar as tubes of different types are concerned, since each type was suitable for use only with its own particular circuit.

An object of this invention is to provide a flame detector system that is universally suitable for use with any selected one of the presently available commercial tubes of different types including ultraviolet tubes, as well as phototype tubes.

Another object is to provide a safe and reliable self-checking flame detector of improved construction.

The present invention provides a multimode self-checking flame detector system which is equally suitable for use with any type of device that is sensitive to rays or waves emitted by the flame being monitored. This is accomplished by a novel amplifier control circuit that accepts the output of any one of several different drive circuits provided therefor, that corresponds to the selected type of tube. Preferably a common circuit module, a circuit board for example, is provided with an individual drive circuit for each of several different types of tubes, and selective switching means that can be set to transfer the output of the so selected tube and its own drive circuit to the input of the common amplifier control circuit.

The amplifier control circuit comprises means for indicating the "on-off" condition of the flame, as well as means for self-checking the circuit itself. The flame is "viewed" by the tube which operates its drive circuit in response to the condition of the flame. The checking circuit periodically energizes a shutter to blind the tube and then checks the output of the amplifier circuit for the so simulated flame loss. If so, the shutter is opened, and normal operation restored. If not, an alarm indicates such malfunction.

The circuit board includes a wave shaping circuit for use with the ultraviolet type tube drive circuit that is selected. In case a phototype tube is selected, its drive circuit is switched into direct connection with the amplifier control circuit, so that the wave shaping circuit is excluded therefrom.

The system comprises self-checking means which automatically operate an alarm in case of any malfunction of the amplifier control circuit, as well as when the flame being monitored becomes extinguished.

In operation the amplifier circuit receives a flame responsive signal from the selected tube of the photoelectric type, or one of the ultraviolet types, through its corresponding drive circuit. The output of such amplifier circuit controls signals that indicate the "on-off" state of the flame, as well as means including a solid state circuit for checking the proper functioning of the system. The flame space is viewed by the selected tube and operates its drive circuit in response to the "on" - "off" state of the flame. Such drive circuit, in turn, operates the amplifier circuit in response to the conditions thereof. The operation of the selected tube is automatically self-checked by the checking circuit that periodically energizes a shutter to blind the tube, and then check the output of the amplifier circuit for the simulated flame loss. If there is no malfunction, the shutter is removed, and normal operation restored; if not, an alarm indicates such malfunction.

The invention provides in the control circuit means for adjusting the sensitivity of the amplifier circuit to the response of the tube and, hence, the flame condition; and means for adjusting the amplifier for varying background flame conditions which are independent of each other. This sensitivity adjustment means relates to the magnitude of response of the amplifier circuit to the response of the selected tube. The sensitivity is adjustable, such that for high sensitivity, the magnitude of response of the amplifier would be greater for a given response of the selected tube than it would be for low sensitivity. Also means are provided for the external selection of two independent sensitivity settings. Another feature comprises means for background adjustment which is externally programmable. This background adjustment means relates to the ability of the amplifier to discriminate between the "on" state of the flame being monitored and the radiation present due to adjacent flame. Further provision includes "Flame On" and "Flame Out" time delay means that are independent of each other.

The self-checking circuit is built into the amplifier control circuit. Solid state circuitry assures reliable operation, and compact construction, as well as ease of repair. Such features and advantages are in addition to the acceptance by the invention of a selected type of ultraviolet tube, or a phototube; which renders the system of virtually universal use, insofar as the particular type of flame sensitive device that is selected is concerned.

The above brief description, as well as further objects, features and advantages of the present invention will be more fully appreciated by reference to the following detailed description of a presently preferred embodiment, when taken in connection with the accompanying drawings.

FIG. 1 is a block diagram of a multimode self-checking flame detector system illustrative of the invention.

FIGS. 2, 3 and 4 are truth tables illustrative of the operation of the A-D converter, the flip flop and time delay, respectively.

FIG. 5 is a circuit diagram of the DC and AC power supplies and wave shaping circuit.

FIGS. 6A and 6B are circuit diagrams of two different ultraviolet tube drive circuits.

FIG. 7 is a circuit diagram of the amplifier control circuit.

FIG. 8 is a circuit diagram of the self-checking circuit.

FIGS. 9 and 9A are diagrams of the AC switch By-directional thyristor and gating circuits.

FIG. 10 is a fragmentary view mainly in side elevation of the flame viewing scope.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1 the system illustrated comprises four major circuits PC17, PC18, PC19 and PC20 which generally correspond to the respective circuit boards or module in which they are container. The circuit boards per se are omitted to simplify and reduce the drawings. The areas of the major circuits are outlined by dotted lines.

The circuit PC20 contains one type of ultraviolet tube drive circuit 22, and another different type of ultraviolet tube drive circuit 24. A common wave shaper 28 is provided for both of the ultraviolet tube drive circuits 22 and 24, being connectable to a selected one of them by means of a two-way switch SW1 having terminals 11 and 13 connected to the circuits 22 and 24, respectively. The proper AC voltages are provided by the corresponding drive circuit to the ultraviolet tubes 32 and 34, respectively, connected thereto.

The wave shaper 28 takes the output from either of ultraviolet drive circuits and converts the AC voltage to a DC current, compatible with the input requirements of main amplifier 36 of circuit board PC18. Phototube 38 is driven by a DC voltage and the signal goes directly to the amplifier 36 when two-way switch SW2 is set to contact terminal 37. However, when it is desired to use one of the ultraviolet tubes 32, 34, switch SW2 is set to contact terminal 39, isolating lead 26 to the phototube 38.

Circuit PC20 also contains a DC power source 21 having an AC input 23; and positive, ground and negative DC outputs 25, 27 and 29, respectively; as well as AC inputs 31 and 33, respectively, to the drive circuits 22 and 24.

Circuit PC18 contains a main amplifier 36, and "Flame On" and "Flame Out" time delay circuits 40 and 41. This circuit PC18 gives an analog output to indicate relative flame strength, and also a delayed logic output "Q" of flip-flop (FF2) circuit 45 which is "1" when there is a flame present, and an "0" when no flame is present. The delays when going to the "1" and to a "0" are independently adjustable. Accordingly, the delay when going to the "1" is adjusted by a potentiometer 7-R30 of FIG. 7, and the delay when going to a "0" is adjusted by potentiometer 7-R27 of FIG. 7.

As shown in FIG. 7, main amplifier 36 contains background adjustment potentiometer 7-R19 and sensitivity adjustment potentiometer 7-R15 and 7-R16. The analog output of the amplifier 36 is indicated by a meter 42, and fed to an A-D converter 44 the outputs of which are "NF" and "F," of FIG. 2. With a flame present, "F" ="0," and with no flame present, "NF"="0." Such two outputs go to the "S" and "R" inputs of flip-flop (FF1) circuit 46, the outputs of which correspond to the truth table, FIG. 3.

Assuming the main amplifier 36 receives a signal responsive to the "sighting" of a flame by the selected tube (32, 34 or 38, "F" would go to "0," and "NF" would be "1." This would make "Q" and "Q" of FF1 circuit 46 become "0" and "1," respectively. With an "0" input to time delay (TD1) circuit 40, its output ("0") would become "1." The output of time delay (TD2) circuit 41 would go to "0" when time t=T2, i.e., T2 seconds after its input went to "1." This would make the inputs to flip-flop (FF2) circuit 45 become "1" and "0" on "R" and "S", respectively, thereby causing the output "Q" of FF2 circuit 45 to go to a "1." Consequently, TD2 circuit 41 controls the "Flame On" delay.

When the selected tube ceases to "see" the flame, "F" and "NF" become "1" and "0," respectively; and "Q" and "Q" of FF1 circuit 46 become "1" and "0." When the input to TD2 circuit 41 goes to "0," its output goes to "1," so that now the inputs to FF2 circuit 45 are both "1" and its output "Q" stays at "1." But the input to TD1 circuit 40 goes to "1" at the same time, so that after T1 seconds, its output goes to "0." This puts a "0" on the "R" input to FF2 circuit 45 which causes its input "Q" to go to "0." Thus, TD1 circuit 40 controls the "Flame Out" delay.

Circuit PC17 comprises circuit means for automatically checking the proper operation of the system. The automatic checking is accomplished by periodically energizing a shutter 48 in scope 50, FIG. 10, to blind the flame detector tube T, and then checking that the output of the amplifier circuit 36, (FIG. 1) indicates the simulated flame loss. If there is flame loss, the shutter 48 is de-energized, and normal operation is restored. If for any reason the amplifier circuit 36 does not respond to the simulated flame loss, the shutter stays energized, and an alarm device 100 is activated to indicate the malfunction.

Time delay (TD3) circuit 52 controls the frequency of the checks. If there is no flame present ("Q" of FF1 circuit 46 is "0") when TD3 circuit 52 times out, it will go to "0" momentarily; but since the "R" input to FF3 circuit 54 is also "0," the output of FF3 circuit is unknown and "Q" may go to "1" momentarily and pulse the checker solenoid 56, FIG. 10. This happens whenever time t= nT3, where n is an integer from 1 to ∞.

Assuming a flame F, FIG. 10, has just been initiated, the "Q" output from circuit PC18 (FF1 circuit 46) has just gone from "0" to "1." When TD3 circuit 52, FIG. 1, has timed (t= nT3), its output goes to "0," thereby changing the status of the output of FF3 circuit 52, so that "Q" is "1" and "Q" is "0." The "Q" output goes to Out-4 circuit 58, which causes the checker to be energized and also starts TD4 circuit 60.

If the output of the flame detector goes to "0" to indicate no flame before TD4 circuit 60 times out, then FF3 circuit 52 is reset ("R" is "0," "S" is "1") and "Q" of FF3 circuit 52 goes back to "0," de-energizing TD4 circuit 60 and the checker; and "Q" of FF3 circuit 52 goes to "1" to start TD3 circuit 52 again. Since the shutter 48, FIG. 10, is deenergized as soon as the amplifier responds to the loss of flame, the main output of circuit PC18, that is "Q" of FF2 circuit 45, does not respond because there is not enough time to time-out TD1 circuit 40.

If, on the other hand, the detector does not pick up the loss of flame, TD4 circuit 60 times out, giving a "0" at "S" of FF4 circuit 62 ("R" is still "1"), so the "Q" output FF4 circuit 62 goes to "1." This "1" goes to Out-3 circuit 64 to give an unsuccessful check alarm by activating device 100, and also to multivibrator (MV1) circuit 66. This starts MV1 circuit 66 oscillating and this signal is used to flash a "Main Flame On" lamp 101 through Out-2 circuit 68, thereby providing another indication of the unsuccessful check. The signal to a Main Flame Relay coil 102 continues undisturbed.

The circuit continues in such state until the amplifier circuit gives a no flame signal ("Q" of FF1 circuit 46 is "0"). When this happens, FF3 circuit 54 is reset as before. "Q" of FF3 circuit 54 going to "0" causes the output of TD4 circuit 60 to go to "1," so that "R" and "S" of FF4 circuit 62 are "0" and "1," respectively; causing "Q" to go to "0." This stops MV1 circuit 66 from oscillating and also de-energizes Out-3 circuit 64 and thereby the alarm device 100.

The output circuits (Out 1-4) 70, 68, 64 and 58 are used to increase the current output capability of the low power solid state circuitry to enable relatively high powered devices such as lamp 101, relay coil 102, solenoid 56 (FIG. 10) and alarm device 100 to be activated.

The circuit PC19 contains AC switching Bi-directional thyristors 74, 76, 78 and 80; and gating circuits 72, 82, 84 and 86. The Bi-directional thyristors drive the external AC operated devices, such as the checker solenoid 56, FIG. 10. The gating circuits receive logic level signals from circuit PC17 and convert them to the proper form to gate the Bi-directional thyristors.

The blocks shown in FIG. 1 correspond substantially to the actual circuitry of the flame detector, and the following description relates to such actual circuits.

UV TUBE DRIVE CIRCUITS

Circuits 22 and 24, FIG. 1, match the specific characteristics of each different type of UV detector tube used therein. These circuits provide the proper drive voltages for the selected tube and also provide a pulse to the wave shaping circuit 28 each time the selected UV detector tube fires.

In FIG. 6A there is shown a drive circuit 22 for use with a unidirectional gas discharge tube 32. This tube 32 conducts only in one direction. When voltage E across AC voltage source T1 is positive and tube 32 conducts, current flows through resistor 112, causing a voltage to appear between lines 114 and 115. Due to the action of capacitor 111 in the circuit the voltage across resistor 112 is of a pulse nature. This pulse is coupled to the wave shaping circuit (FIG. 5) by leads 114 and 115, (FIG. 6A) being connected to leads 108 and 109, FIG. 5, respectively. Resistor 110 serves to discharge capacitor 111 during the time voltage E across AC voltage source T1 is negative and the UV tube 32 is not conducting.

A drive circuit 24 for use with a Bi-directional gas discharge tube 34 is shown in FIG. 6B. This type of tube can conduct in either direction. If the voltage across voltage source T1 is sufficiently high, positive or negative, and UV tube 34 conducts, current will flow through resistor 119. This will cause a voltage across resistor 119 with a polarity depending on the direction of the current flow through tube 34. This voltage is rectified by a diode bridge 120 and applied to the wave shaping circuit of FIG. 5 by connecting leads 121 and 122, FIG. 6B, to leads 108 and 109, FIG. 5, respectively.

WAVE SHAPING CIRCUIT

The circuit 28, FIG. 5 comprises a silicon controlled rectifier Q1, resistors R6 through R13, capacitor C5 and diode bridge D5.

The voltage across the rectifier Q1 is a full wave rectified AC developed by diode bridge D5. Whenever the rectifier Q1 is fired by a pulse from one of the drive circuits it draws current through resistors R8 and R9 and capacitor C5. This charges capacitor C5. During the time when the rectifier Q1 is not conducting, capacitor C5 discharges through resistors R9, R8, R10 and R11, so that the magnitude of the voltage across capacitor C5 is proportional to the period of time the rectifier Q1 is conducting.

The voltage across capacitor C5 is divided down by resistors R11 and R10 and then applied through resistors R12 or R12 and R13 to the input of the amplifier circuit 36 through leads 61, 62 and 63. Resistors R12 and R13 convert the voltage across capacitor C5 to a proportional current which is the proper input to the amplifier circuit 36.

The silicon controlled rectifier (SCR) Q1 is provided because the current pulses produced by each tube are different and may vary with the intensity of ultraviolet light and/or other factors. But, if a pulse is present, it will fire such SCR Q1, which will always conduct for the whole half cycle and the magnitude of the current it draws is fixed by the voltage of the secondary winding 90 of transformer T2, and the values of the resistors in its anode circuit. Therefore, whenever the selected ultraviolet tube fires, uniform pulses appear across capacitor C5, so that its voltage is proportional to the number of pulses per second.

PHOTOTUBE DRIVE CIRCUIT

Phototube 38, FIG. 1, is connected in a series circuit from the negative terminal 29 of the power supply 21 directly to the input of the amplifier circuit 36 through lead 26 to contact 37 of switch SW2.

D.C. POWER SUPPLY

This circuit 21, FIG. 5, comprises a secondary winding 92 of the transformer T2, diode bridge D8, resistors R14 through R16, capacitor C6 and zener diode D6 and D7. It provides D.C. voltage of ±12 volts, for example, to the flame detector circuits. The output of winding 92 of transformer T2 is rectified by diode bridge D8. The output of the diode bridge D8 is filtered by resistor R14 and capacitor C6. The voltage across capacitor C6 is then reduced by resistor R15 and regulated by zener diodes D6 and D7.

MAIN AMPLIFIER

The main amplifier circuit 36 (FIG. 7) comprises substantially all of the circuit to the left of transistor 7-Q3; and is a current-to-voltage converter. The input to this circuit is a current (I in) applied to lead 104, and its output is a voltage (E out) appearing at lead 105 proportional thereto. This is represented by the following formula:

E out = -(I in) (7-R10) (X), where X can be varied from 1-26 to provide the sensitivity adjustment, by adjusting either resistance 7-R15 or 7-R16. This provides two independent adjustments which can be programmed or selected externally by energizing or de-energizing relay 7-RL1 by respectively closing or opening switch 106.

The main amplifier includes an operational amplifier A1 with field effect transistor 7-Q1 used to provide high impedance input stage. Transistor 7-Q2 and resistors 7-R50, 7-R48, and 7-R47 supply a constant source current for transistor 7-Q1. Resistor 7-R36 and capacitor 7-C14 serve as an input filter for the input signal connected to line 104. Resistors 7-R34 and 7-R35 are the load resistors for the input transistor 7-Q1. Resistors 7-R37, 7-R40 and 7-R38, 7-R41 comprise two voltage dividers which reduce the output signal of the input stage to a suitable level for operational amplifier A1. Resistor 7-R45 and diodes 7-D1 and 7-D2 serve to limit the voltage appearing on line 105. Resistor 7-R10 and capacitor 7-C13 are the feedback network for the combined amplifier and field effect input stage while resistors 7-R43, 7-R44, 7-R42 and capacitors 7-C11, 7-C10 and 7-C12 serve to bias and compensate the operational amplifier A1. Diode 7-D3 and resistor 7-R46 establish a stable reference voltage for background adjustment potentiometers 7-R19 and 107, the output of which are applied to the non-inverting input of the main amplifier through resistor 7-R39, and which is filtered by capacitor 7-C9. Resistor 7-R18 in conjunction with sensitivity potentiometers 7-R15 and 7-R16 form two voltage dividers which supply the feedback voltage to the feedback network 7-R10 and 7-C13. Resistor 7-R49 is used to drive a meter to provide a visual indication of flame condition.

A-D CONVERTER

The A-D converter circuit 44, FIG. 7, includes transistor unit 7-Q3 and resistors 7-R23, 7-R24 and 7-R25 connected as shown. Such transistor unit 7-Q3 is of the high gain dual type comprising transistors A and B connected in the differential mode. With the input to transistor A of the transistor unit 7-Q3 negative, the transistor A is cut off and its collector is high. This is the "F" output shown in FIGS. 1 and 2. Transistor B is on, and its collector is low (almost ground). When the input to transistor A of the unit 7-Q3 goes positive, it turns on, its collector ("F") goes low, and the collector of transistor B ("NF") goes high.

FF1 CIRCUIT

The FF1 circuit 46, FIG. 7, includes two cross connected Nand gates 7-G1A and 7-G1C.

TD1 CIRCUIT

The TD1 circuit 40, FIG. 7, includes resistors 7-R26, 7-R27, 7-R28 and 7-R32, capacitor 7-C7, diode 7-D4 and transistor 7-Q4 connected as shown. When the input to this circuit through resistor 7-R26 goes high, capacitor 7-C7 starts to charge through resistors 7-R26 and 7-R27. The gate of transistor 7-Q4 is biased by logic gate 7-G1B at approximately 6.5 volts. When capacitor 7-C7 charges to the bias voltage of transistor 7-Q4 the transistor breaks down, discharges capacitor 7-C7 through resistor 7-R28 and pulls the input to logic gate 7-G1B low. Diode 7-D4 is used to discharge capacitor 7-C7 in case the input to the time delay goes back to zero before the time delay has timed out. This causes instant reset so that the timer will always provide the full time.

TD2 CIRCUIT

The TD2 circuit 41, FIG. 7, includes resistors 7-R29, 7-R30, 7-R31 and 7-R33, capacitor 7-C8, diode 7-D5 and transistor 7-Q5 connected as shown. The operation of this circuit is similar to that described above for TD1 circuit 40.

FF2 CIRCUIT

The FF2 circuit 45, FIG. 7, includes gates 7-G1B and 7-G1D which are connected and operate similarly to the FF1 circuit 46 as described above.

FF3 CIRCUIT

The FF3 circuit 54, FIG. 8, includes gates 8-G1A and 8-G1B which operate as described above re: the FF1 circuit 46.

TD3 CIRCUIT

The TD3 circuit 52, FIG. 8, includes the components to the left of resistor 8-R5 and capacitor 8-C3. The operation of this time delay is substantially similar to that of TD1 circuit 40, except that a voltage divider comprising resistors 8-R3 and 8-R4 is used to bias the gate of transistor 8-Q2; and transistor 8-Q1 is used to amplify the capacitance of capacitor 8-C2. The circuit is such that for similar resistances and capacitances, the de/dt in the first case will be smaller by a value of approximately 1/β, β being the current gain of transistor 8-Q1. Diode 8-D1 discharges capacitor 8-C2 through resistor 8-R2 when transistor 8-Q2 breaks down. Diodes 8-D8 and 8-D9 act to compensate for leakage in capacitor 8-C2 at elevated temperatures.

TD4 CIRCUIT

The TD4 circuit 60, FIG. 8, comprises resistors 8-R6, 8-R7 and 8-R8, capacitor 8-C4, diode 8-D4 and transistor 8-Q3 connected as shown. Operation of this circuit is similar to that to TD1 circuit as set forth above.

FF4 CIRCUIT

The FF4 circuit 62, FIG. 8, includes gates 8- G1C and 8-G1D, and its operation is similar to that of the FF1 circuit 46.

MV1 CIRCUIT

The MV1 circuit 66, FIG. 8, includes resistors 8-R9 through 8-R14, capacitors 8-C5 and 8-C6, diodes 8-D5 and 8-D6, transistors 8-Q4 and 8-Q5. The circuit 66 is one that is known for multivibrators, except that it can operate only when the output of gate 8-G1C is high. With the output of gate 8-G1C low, transistor 8-Q5 is cutoff and its collector, the output of the MV1 circuit 66 is high.

OUT 1-4 CIRCUITS

OUT 1, 2, 3, 4 circuits 70, 68, 64 and 58, FIG. 1, consist essentially of Nand gates 8-G2D, 8-G2C, 8-G2B and 8-G2A, respectively, FIG. 8. Such gates are used to drive the relays 9-RL1, 9-RL2, 9-RL3 and 9-RL4, FIG. 9.

GATING CIRCUITS 1,2,3,4

As shown in FIGS. 9 and 9A, such circuits comprise reed relays 9-RL1, 9-RL2, 9-RL3 and 9-RL4 and diodes 9-D1, 9-D2, 9-D3 and 9-D4. Such reed relays are operated by the outputs of the gates in FIG. 8, and supply the gate signal for Bi-directional thyristors 9-Q1, 9-Q2, 9-Q3 and 9-Q4 which correspond to 74, 76, 78 and 80, FIG. 1. The Bi-directional thyristors, in turn control the operation of device alarm 100, lamp 101, relay 102 and solenoid 56, FIG. 1, as will be clearly understood by those skilled in the art.

SUMMARY OF ADVANTAGES

To summarize, the invention includes the following important advantages and/or features:

1. The system will accept several commercially available ultraviolet or phototube.

2. Background and sensitivity adjustment as defined above are independent of each other whereby a change in one has no effect in the action of the other one.

3. Independent sensitivity adjustments are externally selectable as by switch 106, FIG. 7, which can be located some distance from the flame detector.

4. Background adjustment is externally programmable, as by potentiometer 107, FIG. 7, which also can be located some distance from the flame detector.

5. The system is composed essentially of solid state devices including outputs.

6. "Flame On" and "Flame Out" time delays are independent of each other.

7. The checker circuit constitutes a component part of the system.

A latitude of modification, change and substitution is intended in the foregoing disclosure and in some instances some features of the invention will be employed without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the spirit and scope of the invention herein.