IR light detector Alarm - help needed

[piscesguy]

I am trying to create a pulsed IR light detector alarm. I have experimented with a few circuits and want to try to modify a commonly available circuit for the project. I have built a circuit based on the attached diagram. The circuit works very well but only detects 'dc' light. i.e. it doesn't detect my pulsed IR led (125Hz). I have substituted the phototransistor with an infrared photodiode coupled to a bc109 transistor.

my question is can this circuit be modified to block 'dc' light (say with a capacitor) and use the pulsed AC output of the photodiode/transistor combo to activate the alarm? I hope you understand my explaination.

I am a novice at this but enjoying learning, i just need a few pointers to help me.



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piscesguy
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[piscesguy]

it doesn't detect my pulsed IR led (125Hz).

It does detect the pulsed IR i've re-tested it! but I still need to block 'dc' light signals from acivating the alarm.

Can any one explain how to do this?

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[Simon (Webmaster)]

Hi

As you say, I think a capacitor should work to block constant light signals from the circuit and only allow pulses through. Try a small (0.01uF?) capacitor in series with the base connection of Q1. You might need a pull down resistor (100k?) from the Q1 side of the new capacitor to ground (negative side of battery).

Note the values with questions marks - you might need to experiment with the values, particularly the capacitor value. Obvoisly the circuit might not work if a strong constant light source saturates the sensor input so the pulsed light cannot be detected.

Thinking again you might still have a problem with the circuit triggering when the constant light source switches on / gets brighter, but it shouldn't keep the circuit triggered after the first activation.

[piscesguy]

[quote=As you say, I think a capacitor should work to block constant light signals from the circuit and only allow pulses through. Try a small (0.01uF?) capacitor in series with the base connection of Q1. You might need a pull down resistor (100k?) from the Q1 side of the new capacitor to ground (negative side of battery).[/quote]

Thank you for your comments. I have tried a few small capacitors as suggested, this blocks ambient & the pulsed IR. I have tried 0.01, 0.1 & uF values but when I do this there is no voltage on pin 2. I have tried removing C1, adjusting P1 and reducing R1. Tried the pull down resistor but as I understand it..thats to make sure theres no voltage on pin 2 when the input from the transistor is off?. I've checked the base side of Q1 before the capacitor and there's around 8Volts there (under lighted conditions) but nothing at the Q1 side of the capacitor.

so it seems the ac frequency is not 'strong' enough to bridge the capacitor

Is that because the frequency i'm using is low (125hz)? does this mean I need a much smaller capacitor.? am I making sense..I am at the limit of my understanding of electronics.

I think I really need a circuit design with a wide bandpass filter with lower and upper cut off levels of 80 and 700hz. Would I be better incorporating an op amp based bandpass filter in place of Q1?


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