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Can I Use a LDR (light-dependent resistors) as a Simple Switch

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Feb 17, 2022
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I have one of those sound chip greeting card circuits. The card plays the message when a momentary contact switch is depressed and released. I also have an inexpensive "driveway alarm" that has a sensor that triggers a receiver to make a beep and flash a LED.

I want to use the driveway alarm to activate the sound card.

I would like to keep things simple and not hack the alarm boards. I am thinking that I could just tape a LDR right over the top of the LED and hook the LDR leads into the sound card circuit in parallel with the momentary contact switch. When the LED flashes on, I am in hopes that the LDR will drop to zero resistance and act as the momentary contact manual switch to activate the sound card.

What are your thoughts and recommendations?

Thank you, Barry! I like the “on board” signal processing. I hope you help me out by describing exactly how to use this device in the arrangement I described.

Let’s start with the light detecting “lense” is in proximity of the alarm LED. On the other end is the momentary switch activating the sound card. Where would I connect the three legs of this device and would I have to provide power?

Barry, I have attached a sketch that is NOT a schematic but hopefully conveys my needed solution.
Think of the sound card as a free standing device with its own power supply, etc. and has as input a manual monetary contact switch/
Think of the "Driveway" Sensor/Receiver as a free standing device that has as an output a LED
I want a "black box" free-standing device that "reads" the LED emitted light. processes the On/Off status with a photologic device that "reads" the LED and then outputs by acting like a switch to the Sound CArd.


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On your sound card switch, one terminal is PROBABLY ground (you'll have to figure that out). Connect the ground terminal of the device there. Connect the output to the other switch terminal. Connect VCC of the device to the battery (+).

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