NE555: Smoke Detector Circuit

Michael Black2

Part Number: NE555

Hello,

I am having some trouble understanding how this smoke detector circuit using the NE555 works.

Source: https://www.elprocus.com/smoke-detector-circuit-using-ic-555-timer/

1. I don't know enough about smoke and phototransistors - does the smoke completely block the light from the LED (causing the signal to be steady), or does it only partially block the light (causing a more PWM-like signal)?

2. Are we driving the speaker by rapidly toggling the output, p3? i.e. quickly toggling between two of the conditions (one that causes output to go low, one that causes output to go high) in Table 1 from the NE555 Datasheet, or is something else going on?

Let me know if you need clarification on anything.

Also, if you have any design resources that would help me, that would also be apprecitated.

over 4 years ago

0 Chris Featherstone over 4 years ago

TI__Mastermind 25210 points

Michael,

I will need two days to look this over. Thank you for your patience.

+1 Chris Featherstone over 4 years ago

TI__Mastermind 25210 points

Michael,

I have put together a simulation for you (see very bottom of post to download). When there is no smoke the LED is driving the transistor base with a voltage to turn it on. When the transistor is on the collector to emitter voltage is approximately 0V since the source is tied to ground. This in turn causes the reset pin to be 0V and the device has no signal on it's output.

When the LED is covered with smoke it shuts off the transistor and the reset pin is pulled up to 9V. This cause the output to cycle with a timing chosen by the RC values.

555 Smoke Detector.TSC

0 Michael Black2 over 4 years ago in reply to Chris Featherstone

Intellectual 350 points

Just to make sure I understand completely, The RC values cause The THRESH and TRIG pins to oscillate between <1/3Vcc and >2/3Vcc (rows 2 and 3 on Table 1) whenever the RESET pin is high - is that correct?

+1 Chris Featherstone over 4 years ago in reply to Michael Black2

TI__Mastermind 25210 points

The RC time constant dictates how long it will take the capacitor to charge and discharge between 1/3Vcc and 2/3 Vcc. The equations for the the high time and low time are shown on page 16. Here is a snippet

Clock & timing

Clock & timing forum

NE555: Smoke Detector Circuit