Complete Proteus Circuit Heat Activated Alarm

Circuit Diagram for "Heat Activated Alarm"

I am working on a project in which I have to simulate a heat activated alarm. I have attached two images. One of which is a Proteus circuit image that i have made. When i start its simulation the speaker attached in the circuit doesn't play any sort of sound. Even if i raise the temperature to 150 degrees. The circuit diagram that i am following for this circuit has also been attached. I guess there must be some thing that i am missing. Can anyone help ?? Regards, -Rehan

  • \$\begingroup\$ What happens if you pull 555's pin 4 to Vcc? Disconnect the transistor's collector first. You effectively disconnect the sensor stage and the 555 should start to oscillate if its configuration is correct. \$\endgroup\$
    – jippie
    May 5, 2013 at 12:51
  • \$\begingroup\$ @CamilStaps I have disconnected the sensor stage as you said and run the simulation but still the speaker plays no sound. \$\endgroup\$ May 5, 2013 at 14:45
  • \$\begingroup\$ You must be talking to @jippie, let me mention him for you. \$\endgroup\$
    – user17592
    May 5, 2013 at 14:46
  • \$\begingroup\$ Why is there a 8k2 resistor in the Vcc power supply line? Please clean up your Proteus circuit diagram (make it look more like the original diagram). To be honest, it looks a bit like chaos now and is pretty hard to read for us. \$\endgroup\$
    – jippie
    May 5, 2013 at 14:49

2 Answers 2


Test the parts separately.

1) Pull the 555 pin 4 to +V with a 1 kilohm resistor. Does the noise start? If not, investigate the 555 in more detail. But assuming yes:

2) Pull the sensor transistor base to 0V (likewise). Does it sound? If not, check the transistor and its connections. If yes:

3) Measure Q1 base voltage at different temperatures. If it increases with temperature, you have the wrong type of thermistor... To fix, interchange the thermistor with the variable resistor. Find the base voltage at which the sounder starts to work, and adjust R (the variable R) until you get that voltage as you approach the right temperature.

If you're still stuck, add these results to the question...


For starters: move R3 from pin 8 to pin 7 and connect pin 8 directly to 12VDC.

With the current configuration pin 7 is tied to Vcc and it prevents the 555 from oscillating.

  • \$\begingroup\$ Could you explain why? \$\endgroup\$
    – user17592
    May 5, 2013 at 15:34
  • \$\begingroup\$ Because the lower diagram is different in that respect? \$\endgroup\$
    – jippie
    May 5, 2013 at 15:37

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