Hey,
I've got a threshold detector setup using an MC3407 as a comparator. It outputs ~9V when the input goes above 0.5V. Currently the output drives a 10mA red LED. However, since this is for sound often the light flickers to fast to see. So I'd like to have the time at which the LED is illuminated for after the threshold is reached be extended to around 1/3 to 1/2 of a second. I've attached a circuit of how I think this can be done. However, I'm not yet too sure whether the OPAMP can drive something like this, and if it can how would I go about choosing the correct capacitive and resistive values. (The LED resistor is easy to pick).
Could I say that when the voltage rises to +9V then assume 0.7V drop across the diode, so the circuit acts like an RC circuit and charges the capacitor quickly up to 8.3V. Then once the voltage from the OPAMP drops to 0V. The diode ceases conducting and the cap discharges at a rate of 2PI*C*RES_LED?
My guess is I would need a capacitor of about: 0.4seconds/ (2PI*Res_LED) which gives me about 94uF (LED uses an 680ohm resistor at 9V nominally).
Thanks,
TrevorP
I've got a threshold detector setup using an MC3407 as a comparator. It outputs ~9V when the input goes above 0.5V. Currently the output drives a 10mA red LED. However, since this is for sound often the light flickers to fast to see. So I'd like to have the time at which the LED is illuminated for after the threshold is reached be extended to around 1/3 to 1/2 of a second. I've attached a circuit of how I think this can be done. However, I'm not yet too sure whether the OPAMP can drive something like this, and if it can how would I go about choosing the correct capacitive and resistive values. (The LED resistor is easy to pick).
Could I say that when the voltage rises to +9V then assume 0.7V drop across the diode, so the circuit acts like an RC circuit and charges the capacitor quickly up to 8.3V. Then once the voltage from the OPAMP drops to 0V. The diode ceases conducting and the cap discharges at a rate of 2PI*C*RES_LED?
My guess is I would need a capacitor of about: 0.4seconds/ (2PI*Res_LED) which gives me about 94uF (LED uses an 680ohm resistor at 9V nominally).
Thanks,
TrevorP
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