Abstract:
A portable unit indicates the precise time to close a fireplace damper door. The unit comprises a housing including a power source, and an electronic circuit, and an external heat sensor. The arrangement is such that the heat sensor is placed near the firebox and senses that a fire has been lit by the heat given off. The electronic circuit then stores this information and waits until the heat is no longer present. At this time, a fire extinguished indication is displayed, and a time delay circuit activated. If a fire is restarted before the delay circuit times out, the fire extinguished status is cleared and the process started over. If a fire is not restarted, and the delay circuit does time out, then a audible indication warns that the fireplace damper can now be manually closed to save energy.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a portable device which indicates exactly when to manually close a fireplace damper door. Because people often forget to close the damper door and/or do not know the right time to close it, heat escapes up the chimney flue. This results in reduced home heating efficiency. 
     Presently, there are indicators that tell whether the damper is open or closed, all of which use mechanical levers and screws directly coupled to the damper for means of indication. Also, these devices only indicate an open or closed status. They do not tell precisely when to close the damper. example, U.S. Pat. No. 4,928,668 discloses a device including an indicator lamp which is electrically coupled to the damper door opening lever. When the damper is open, the light is on, and vice versa. One still could leave the damper door open if they were not looking directly at the fire and the present indication. Also, one would still not know exactly when to close the damper for maximum heating effiency. Another such device is disclosed for example, in U.S. Pat. No. 2,393,482 wherein a meter indicates damper position. Here the same problems exist. Both devices are costly and very difficult to install. 
     SUMMARY OF INVENTION: 
     The present invention is directed to a portable, self contained, electronic device which indicates exactly when to close a firebox damper door. The device comprises, in combination a heat sensor means, a circuit for establishing a reference or comparison temperature, a circuit for storage of temperature differences, a circuit to detect a change in stored temperature, a fire extinguished indicating means, a reset circuit in the case of a restarted fire, a time delay circuit, and a means for producing an output signal which indicates exactly when to close the damper door. 
     The device works by comparing the temperature in or around the firebox to a reference temperature (house plus offset temperature). If the temperature in the firebox is greater, a fire has been lit. The unit then stores this information and waits until the fire has been extinquished. When this occurs, the firebox temperature goes lower than the reference. A indication of this status is displayed and a time delay circuit is activated. If an increase in firebox temperature occurs, the unit resets itself. If the firebox temperature remains lower and the delay circuit times out, then an indication to warn that the damper door is open is sounded. 
    
    
     DESCRIPTION OF THE DRAWINGS: 
     FIG. 1 is a schematic view of an electronic device according to one embodiment of the invention; 
     FIG. 2 is a schematic view of a similar device according to another embodiment of the invention; 
     FIG. 3 shows the waveform of the change in firebox temperature verses time. 
     FIG. 4 shows the device embodied in a housing; and 
     FIG. 5 shows the device connected to a fireplace. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 5 there is shown a firebox with a damper door located within the firebox. After a fire is lit, one could either forget to close the damper and/or not close it at the right time. However, if the electronic alert device according to the invention and generally shown at 75 is placed near the firebox, and sensor 10 placed into the lower part of the glass doors or screen, then this problem can be avoided. 
     In accordance with the present invention, there is provided a means of giving an audible warning of when to close a damper door. The circuit diagram of a first embodiment of the device is shown in FIG. 1. The overall object of the close damper alert unit is to sense that the firebox is at the right temperature so that one can be warned to close the damper door for maximum home heating effiency. The heat sensor 10 which is placed near or in the firebox is shown in FIG. 1. This heat sensor changes resistance as a function of temperature. If rheostat 11 is placed in series with sensor 10, then a voltage divider is formed when the circuit is connected as viewed. 
     Operational amplifier (op-amp) 19 then compares this firebox divider voltage to a house plus offset voltage. The house potentiometer 16 has a range of from fifty to ninety degrees fahrenheit. The circuit components 12,13,14 and 15 provide a voltage reference which represents the house plus offset temperature at a house temperature of ninety degrees. The circuit components 17 and 18 provide a voltage reference which represents the house plus offset temperature at a house temperature of fifty degrees. When the firebox voltage exceeds the house plus offset voltage, the output of 19 swings from ground to +V. The current flow from 19 then charges capacitor 21 through diode 20. It also sends the output of 37 to swing from +V to ground. As 21 builds up charge, the output of 23 swings from ground to +V. The register 30 and 31 are both cleared (logic 0) prior to any action. However, when the output of 23 swings to +V the register upon the first low to high transition of oscillator 29 shifts from a logic 00 to a logic 10. Upon the second transition of the oscillator the register shifts again from logic 10 to a logic 11. All shifting occurs from right to left. The register, now loaded with a logic count of 11, waits until the firebox voltage goes below the reference. 
     With this occurance, the output of 19 swings to ground and the output of 37 swings to +V. The time delay formed by the discharging of 21 through 22 ensures that 37 is at +V before the register content is changed. Shortly after the voltage from discharging 21 and 22 falls below the voltage reference established by 24 and 25, the output of 23 swings to ground. The register content which is a logic 11 changes to a logic 01 upon the first transition of the oscillator 29. The decoder circuit, formed by gates 32 and 33, swings it&#39;s output to +V. This sends a current flowing through 34 and 35 which fires into the gate of SCR 36. SCR 36, upon receiving it&#39;s firing signal, latches up and allows the fire extinguished light 39 and variable time delay (40,41,42) to become active. On the next pulse from oscillator 29, the register content (30,31) changes again from a logic 01 to 00. The decoder circuit (32,33) then resumes back to it&#39;s original output state of ground. This stops current flowing into the gate of SCR 36. 
     If the firebox voltage should exceed the reference before capacitor 42 reaches it&#39;s threshold voltage set by 48 and 49, then the output of 19 swings to +V which causes 37 to release the latch on 36 and thus to turn off LED 39. It also causes 42 to discharge back to ground through diode 41 and sets the register back to a logic 11 state. It then waits until the firebox voltage falls below the reference at which point the process of detection is started over. 
     If the firebox voltage remains below the reference, and resistor 40 supplies enough charge to capacitor 42 so that the voltage threshold at op-amp 43 is exceeded then the output of 43 swings from ground to +V. 44 and 45 experience this change by sending current to the gate of SCR 46. SCR 46 which becomes latched turns on audible transducer 47. The only way of clearing this indication is by opening switch 55. 
     Other circuit functions include that of 50,51,52 and 53. Here, when battery potential 54 is applied to +V through the closing of switch 55 (the unit is turned on), the output of 53 swings from +V to ground. This action clears register flip flops 30 and 31. 
     Referring to FIG. 2, there is shown an alert device according to another embodiment of the invention for indicating exactly when to close a firebox damper door. As in the previous embodiment, operation of the circuit also detects changes in firebox temperature. However, this circuit uses two heat sensors and functions as follows. As shown in FIG. 2, the heat sensor 10 is placed near or in the firebox. This heat sensor changes resistance as a function of temperature. If rheostat 11 is placed in series with 10 then a voltage divider is formed when the circuit is connected as viewed. Operational amplifier (op-amp) 19 then compares this firebox divider voltage to a house plus offset voltage. The house plus offset voltage is produced by components 12,13,14,15,16,17,18, and 18a. Here, heat sensor 12, which functions in the same manner as 10, is located within housing 75 as shown in FIG. 5. It&#39;s purpose is to automatically read in the house temperature so that no external potentiometers have to be set (16 in FIG. 1). When 12 is placed in series with rheostat 13 a voltage divider is formed. The voltage picked off at the plus terminal of 14 represents the true house temperature. The circuit action of 14, which is a non-inverting op-amp, has a gain potentiometer 17. This sets the offset temperature voltage. Components 18 and 18a set a scale down factor which feeds the negative terminal of op-amp 19. When the firebox voltage exceeds the house plus offset voltage, the output of 19 swings from ground to +V. The current flow from 19 then charges capacitor 21 through diode 20. It also sends the output of 37 to swing from +V to ground. As 21 builds up charge, the output of 23 swings from ground to +V. The register 30 and 31 are both cleared (logic 0) prior to any action. However, when the output of 23 swings to +V the register upon the first low to high transition of oscillator 29 shifts from a logic 00 to a logic 10. Upon the second transition of the oscillator the register shifts again from logic 10 to a logic 11. All shifting occurs from right to left. The register, now loaded with a logic count of 11, waits until the firebox voltage goes below the reference. 
     With this occurance, the output of 19 swings to ground and the output of 37 swings to +V. The time delay formed by the discharging of 21 and 22 ensures that 37 is at +V before the register content is changed. Shortly after the voltage from discharging 21 and 22 falls below the voltage reference established by 24 and 25, the output of 23 swings to ground. The register content which is a logic 11 changes to a logic 01 upon the first transition of the oscillator 29. The decoder circuit, formed by gates 32 and 33, swings it&#39;s output to +V. This sends a current flowing through 34 and 35 which fires into the gate of SCR 36. SCR 36, upon receiving it&#39;s firing signal, latches up and allows the fire extinguished light 39 and variable time delay (40,41,42) to become active. On the next pulse from oscillator 29, the register content (30,31) changes again from a logic 01 to 00. The decoder circuit (32,33) then resumes back to it&#39;s original output state of ground. This stops current flowing into the gate of SCR 36. 
     If the firebox voltage should exceed the reference before capacitor 42 reaches it&#39;s threshold voltage set by 48 and 49, then the output of 19 swings to +V which causes 37 to release the latch on 36 and thus to turn off LED 39. It also causes 42 to discharge back to ground through diode 41 and sets the register back to a logic 11 state. It then waits until the firebox voltage falls below the reference at which point the process of detection is started over. 
     If the firebox voltage remains below the reference, and resistor 40 supplies enough charge to capacitor 42 so that the voltage threshold at op-amp 43 is exceeded then the output of 43 swings from ground to +V. 44 and 45 experience this change by sending current to the gate of SCR 46. SCR 46 which becomes latched turns on audible transducer 47. The only way of clearing this indication is by opening switch 55. 
     Other circuit functions include that of 50,51,52, and 53. Here, when battery potential 54 is applied to +V through the closing of switch 55 (the unit is turned on), the output of 53 swings from +V to ground. This action clears register flip flops 30 and 31. 
     FIG. 3 shows a typical heat curve of a firebox with several key points lettered A to I. The present invention uses this curve by starting off at time zero with the house temperature A at 68 degrees Fahrenheit and firebox temperature I at the same. When a fire is lit, the fire box temperature rises. The reference temperature C, which is the summation of house A and offset B temperatures, forms a final temperature of 73 degrees F. The offset temperature B is set in this example at 5 degrees F. As I rises it passes through point D. Here is when the firebox voltage exceeds the reference and output 19 swings from ground to +V. The register, which is at a logic 11, then waits until the firebox voltage falls below the reference. 
     This occurs when curve I passes through point E. Now the decoder fires SCR 36 which activates the fire extinguished light and starts the variable time delay circuit of 40,41, and 42. 
     If the firebox temperature exceeds the reference before the delay times out, as illustrated by curve G at intersection F, then 37 releases SCR latch 36 and the LED 38 turns off. It also sets the variable time delay back to zero and returns register 30 and 31 back to a logic 11. The process of detecting point E is started over. 
     If the firebox temperature remains below the reference, and the delay curcuit reaches it&#39;s threshold and times out, then at point H an audible transducer is sounded. 
     FIG. 4 illustrates the device of the invention in a housing. It shows the physical location of on/off switch 55, battery source 54, firebox heat sensor 10, house temperature potentiometer 16, fire extinguished light 39, and audible transducer 47.