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T O P I C    R E V I E W
tecnovative Posted - Nov 20 2009 : 02:51:29 AM
This is a simple but useful circuit, thanks for making it freely available. That said I expected the circuit to function in reverse, instead of the LED dimming when airflow is detected I assumed it would brighten. Any suggestions on how to modify the circuit so that the LED brightens with air movement? I tried reversing the input signal to the comparator and it does work but the output barely lights the LED. Also any suggestions on a good simple way to send a consistant output signal to an analog meter or DMM?
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tecnovative Posted - Nov 21 2009 : 9:19:31 PM
quote:
If you swap pin 4 and pin 5 then the LED will light with the same brightness as before except when there is a draft on the sensor.
The comparator is supposed to have some positive feedback (hysteresis) to prevent it from oscillating. If it oscillates then the LED will not turn on and off properly.

Audioguru: I found that I had errored in connecting the regulator. I confused the output and ground pins of U1. With the circuit built to the schematic specs the LED is on until air flow is detected across the sensor and then it switches off. I was expecting a varied LED brightness proportional to the air flow across the sensor, not simply a high or low (on or off) condition. Ironicly with U2 and the sensor connected to the ouput pin of the regulator rather than ground the LED brightness was varied with air flow but it starts out lit and then dims proportional to the air flow. I want the LED to be off until air flow is detected and then increase in brightness proportional to the air flow.
audioguru Posted - Nov 20 2009 : 1:07:55 PM
If you swap pin 4 and pin 5 then the LED will light with the same brightness as before except when there is a draft on the sensor.
The comparator is supposed to have some positive feedback (hysteresis) to prevent it from oscillating. If it oscillates then the LED will not turn on and off properly.

The LM339 is a quad comparator. An LM393 is a dual comparator in an 8-pins case.

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