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on/off light switch/controller

newkid

Aug 4, 2012
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Hi everyone, I'm going to start by saying that electronics are far from my forte, so, please, excuse my ignorance.

I have a motor and I want to build a circuit to control when the motor is turned on/off. I want the motor to be turned on and off based on the movement of a certain object and I plan on using an LED and a photocell to do that. I want a circuit that's set up so that the motor is turned on if that object blocks the LED and it's "dark" and I want it to be off as long as the photocell can detect the light coming off of the LED.

1- I think the voltage coming off of the photocell isn't strong enough to control the motor so I need to amplify that signal (transistor?) but I'm not too sure I know how to do that.
2- how do I make sure that the motor is either on or off without using a digital signal?
3- what other components do I need?

I tried searching on the forum but I couldn't find anything. It is possible, though, that I'm using the wrong terms in the search so, please, feel free to direct me to a thread/an article.

On a side note, what is a good website to buy components off of?

Thanks in advance.
 

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
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Doable but we need the specs on your motor...Operating Voltage and Current. We also need the part number of your photocell and emitter.

Edit: The term "Photocell" is often misused. There are Photo-Resistors (Light Dependent Resistor (LDR)), Photo-Diodes, Photo-Transistors and there are Solar-Cells.
 
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donkey

Feb 26, 2011
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is there a difference between photo resistors and light dependant resistors?
 

CDRIVE

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is there a difference between photo resistors and light dependant resistors?

Not that I know of. In the old days almost no one used the term LDR but Photocell was common. We now have too many optical devices to use a generic term like that. For an LDR it was probably a misnomer anyway. The LDR predates solid state.

BTW, I edited that post to include LDR.
 
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donkey

Feb 26, 2011
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lol it was for my knowledge base, sorry for sidetracking the topic here.

Newkid, as with all projects the more we know the better we can help. I could tell you I have a battery and light bulb and the light is not shining. people could speculate for hours as to the reason. but if i told them it was a 240v light bulb and a AA battery, they could tell me straight away the problem.
give us as much info and pics as you can, or even a simple design.
also tell us what ests you have completed
1- I think the voltage coming off of the photocell isn't strong enough to control the motor so I need to amplify that signal (transistor?) but I'm not too sure I know how to do that.
have you used a meter to check the power? tell us how

and as for buying compnents it depends a whole heap on where you live. Everyone has an opinion on this matter so feel free to google the heck out of companies and find reviews
 

newkid

Aug 4, 2012
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thanks for the replies everyone.

I don't have any components yet. CDRIVE, how do I pick a specific kind of photocell?

I don't know the operating current for the motor because I haven't tested yet, but it's a 12V motor and I'm going to power it with a 12V battery pack. I'll test and let you know.

I thought that the the signal wouldn't be strong enough because the on/off light operated circuits that I've seen online had transistors.
 

john monks

Mar 9, 2012
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If 12 volts is your supply then you could use a 12 volt relay driven by a darlington transistor pair that gets it's signal from a light dependent resistor. And you could use an LED light source and some lenses like what you find in grocery stores.

A relay would be good because you don't know the electrical characteristics of the motor and a relay can be very forgiving.

Do you intend to use a 12 volt battery with a battery charger?
 

CDRIVE

Hauling 10' pipe on a Trek Shift3
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Telling us that it's a 12V motor isn't telling us much. A 12V motor can be anything from a small computer fan motor that draws a few milliamps to a truck starter motor that pulls 400Amps. Please be more specific.
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
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Will there be an issue with ambient light falling on your receiver? It's common in these cases to modulate the transmitted light at a certain frequency, and filter the receiver output to detect that frequency, so that your circuit detects the light from the emitter and is immune to ambient light.

You can probably buy a shop door monitor that has everything you need apart from the capability to drive a motor. It could even have an extra output that's designed to drive an external buzzer or bell, which you could use to switch the motor. If you need to boost the drive capability, relays are cheap and readily available.
 

newkid

Aug 4, 2012
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If 12 volts is your supply then you could use a 12 volt relay driven by a darlington transistor pair that gets it's signal from a light dependent resistor. And you could use an LED light source and some lenses like what you find in grocery stores.

A relay would be good because you don't know the electrical characteristics of the motor and a relay can be very forgiving.

Do you intend to use a 12 volt battery with a battery charger?

thanks, that's very helpful. yes, initially, a battery pack with a battery charger but I'll probably switch to a wall plug with a transformer when the circuit is done.

Telling us that it's a 12V motor isn't telling us much. A 12V motor can be anything from a small computer fan motor that draws a few milliamps to a truck starter motor that pulls 400Amps. Please be more specific.

I'm sorry, but I don't know much.. It's a very small motor from an old car antenna. I picked it because of its size and price. I still haven't had the time to work the motor under load so I don't know how much current it will draw.

Will there be an issue with ambient light falling on your receiver? It's common in these cases to modulate the transmitted light at a certain frequency, and filter the receiver output to detect that frequency, so that your circuit detects the light from the emitter and is immune to ambient light.

You can probably buy a shop door monitor that has everything you need apart from the capability to drive a motor. It could even have an extra output that's designed to drive an external buzzer or bell, which you could use to switch the motor. If you need to boost the drive capability, relays are cheap and readily available.

thanks, that's very helpful! I was worried about ambient lighting but I thought I would get the circuit working first and then try to fix that.
 
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