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Re: sensor circuit

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Hi Albert,

you might substitute the photo transistor with a normal NPN transistor. Apply base voltage which will simulate the light passing to the photo transistor pulling the collector low.

Switching the base to ground will then simulate an obstructed view switching the collector high.

The connected OR-gate will function normally.


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Why do you need to simulate it?

There shouldn't be any need to because the circuit is so simple, just use Boolean algebra to work out what will happen.

Even if you simulate it and it works it doesn't mean it'll work in real life. The simulation program doesn't know the ambient lighting conditions, the brightness of the LEDs or how well the phototransistors are shielded.

Providing the LEDs are bright enough to cause the phototransistors to saturate at a low enough voltage and the phototransistors are well enough shielded from ambient light to turn off enough, it will work.

A simple circuit like this won't be very reliable because the ambient lighting can interfere with it. A real beam break detector circuit pulses the IR LEDs at a specific frequency and has a bandpass filter or PLL (Phase Locked Loop) after the phototransistor to reject constant or mains frequency light sources.

I would use an astable multivibrator (555 timer or logic gates) to pulse the LEDs at about 40kHz and a tone decoder (NE567 or CD4046) for the IR receiver section. I wouldn't bother with a logic gate after the receiver, I'd connect the phototransistors in series. To make it more efficient, I'd connect the LEDs in series.

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Guest -albert-

thanks for the advice...

i have another question if you wouldnt mind answering....the IC im using for the OR gate is 7432 which has an operating voltage of 4.75v-5.25v...would i be damaging it if i series 4 pcs of 1.5V batteries which is 6v? i dont want to build a psu because its going to be used only once

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The absolute maximum voltage rating is 7V so you'll probably get away with it but it isn't recommended. The trouble is the battery voltage will drop below the minimum voltage rating before the the cells fully discharged.

Why are you using an old power hungry TTL lC?

You should use a low current CMOS IC such as the 74HC32 which has the same pin-out as the crappy 7432 but with a wide power supply voltage range of 2V to 6V and a much lower current consumption.

An alternative is the CD4071 but note the different pin-out and voltage range of 3V to 15V.

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