Resistance matching (hobby project)

Discussion in 'General Electronics Chat' started by Brandon\$B, Dec 12, 2013.

1. Brandon\$B Thread Starter New Member

Dec 12, 2013
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Hi guys, it's Brandon here.

So I'm building a cellphone based switch to open and close my home gate(electronic), the original switch is just a push to make contact switch so I started by removed the vibrator from the cellphone and used the custom set vibration signal to power an Light emitting diode(LED) which will then illuminate a light dependant resistor (LDR), which will be amplified and as a result a relay will be controlled, which will will be connected in parallel to the existing push-make switch. The whole aim here is when a specific number is called the phone will vibrate for that number and open the gate. The reason I'm still doing this is to electrically isolate the two circuits because I might have to use the cellphone as, well a cellphone. I don't think this is completely necessary I'm just being redundant.

So my question is will I need to match the resistance of the motor(vibrator) to this resistance of the LED circuit exactly, or can it be higher or lower than that of the motor?

I'm studying electrical engineering at school so I understand everything in a theoretical manor but I am yet to understand it in a practical one.

Additionally I would like to know it there is a simpler way of going about this? I know I'm not using a photocoupler (opto iso) I will upgrade that part at a later stage. I know I can use a GSM based receiver but I want to do this on my own.

Brandon Pillay

2. Alec_t AAC Fanatic!

Sep 17, 2013
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The LED circuit can have a higher resistance than the motor.
I'm puzzled, however, as to why you want to do this project? The LED would need to be very close to the LDR to get it to respond correctly. So if you have to get close, why not just use the push switch and forget the phone?

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3. wayneh Expert

Sep 9, 2010
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I'd add a comparator to sense the motor-on condition, and use the comparator output to control a transistor and then the relay. The comparator has a very high impedance input and will not disrupt normal function of the motor. Very non-intrusive. No load on any of the cellphone circuitry.

If you continue on with the LED approach, consider an optoisolator. It's basically the purpose-built component to do what you described. A matched LED and sensor in one tiny package. I don't know if there is enough voltage across the motor to drive an LED, though.

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4. davebee Well-Known Member

Oct 22, 2008
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The motor must draw a good amount of current, so you'd probably be safe drawing that much or less current to run an LED (or an optoisolator). I wouldn't try drawing more current, because that would tend to have a bad effect on the driving circuitry.

I like the use of an LED, because that way the phone can be completely mechanically as well as electrically independent. You'd just need to make a phone holder that would position the LED to illuminate the detector and to shield external light from the detector.

This sounds like a nice project.

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5. Brandon\$B Thread Starter New Member

Dec 12, 2013
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This is my fault, I didn't explain it well enough.

So there are two phones, phone A and phone B. Phone B has been programmed to vibrate only when the number of phone A calls or attempts to call phone B(custom notification function on phone B) it will vibrate. Then that signal can then be used to control a relay to open/close a switch. This will enable me to control a switch from literally almost around the world by calling phone B with phone A.

Thank you for replying you my message.

6. Brandon\$B Thread Starter New Member

Dec 12, 2013
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Thank you for replying to my message davebee

I will upgrade this device at a later stage from a LED and LDR to a single package this is just a prototype, I will tape the LDR to the LED to maximize illumination and isolate the light source.

7. Brandon\$B Thread Starter New Member

Dec 12, 2013
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Thank you for replying to my post Wayneh

Thanks for that, I haven't thought of that, I will work on that.