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Help designing a remote controlled cupboard lock

Pommie

Well-Known Member
Most Helpful Member
No, you can't use an audio amplifier this way. Look at the link Nigel posted earlier.

Mike.
 

crutschow

Well-Known Member
Most Helpful Member
Basically you use a diode to rectify the AC ring signal to give a DC voltage, and than amplify that DC to drive the door lock solenoid.
So you can generally just replace the microphone with the ring signal connection, in most of those circuits in Nigel's link.

You don't want/need to AC power amplify the ring signal before it is rectified to DC.
 

Asheekay

New Member
Hmm got it. I was thinking that being an electronic device, a cellphone must be sending dc signals to the speaker. Looks like I was completely wrong.

So, the correct approach is this:

Connect the cellphone speaker cords to a full bridge rectifier.

Connect the output of the rectifier to the audio amplifier.

Connect the output of the amplifier to the dc motor.

Am I right? Am I right?
 

Asheekay

New Member
I don't understand either of them :(

And worse yet, I don't hope to find the required components online or where I'm located (a small village)
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Sigh. How else am I supposed to "amplify" the signal then?
There are many different types of "amplifier" used in electronics; audio is just one of them.

What you would be making is really an electronic switch, which turns on or off dependant on the amount of voltage (or lack of it) from the rectifier.

Although it is technically a kind of amplifier, it's not handling audio and such applications where the output is basically an on-off signal is more considered the realm of "logic" circuitry rather than amplification.


Amplification generally means the output is proportional to the input; logic just deals with hi/lo, on/off, true/false or whichever two state names you prefer.

[If you connected a motor to the output of an audio amplifier, you could make it try to oscillate back & forth a tiny amount like a loudspeaker cone movement, which is not what you want].
 

Asheekay

New Member
There are many different types of "amplifier" used in electronics; audio is just one of them.

What you would be making is really an electronic switch, which turns on or off dependant on the amount of voltage (or lack of it) from the rectifier.

Although it is technically a kind of amplifier, it's not handling audio and such applications where the output is basically an on-off signal is more considered the realm of "logic" circuitry rather than amplification.


Amplification generally means the output is proportional to the input; logic just deals with hi/lo, on/off, true/false or whichever two state names you prefer.

[If you connected a motor to the output of an audio amplifier, you could make it try to oscillate back & forth a tiny amount like a loudspeaker cone movement, which is not what you want].
Ah, finally something I can actually understand!

Considering the on-off switch approach, my first idea was to simply rectify the ringing signal, then send the output to a transistor (emitter grounded, base connected to the ringing signal) and use the collector to turn on the motor with a 12v battery.

But then seniors here told me this is the wrong approach.

Hence my next approach was to amplify the ringing signal, then rectify it into dc (so that the audio oscillations are increased, then converted into dc) and send that output to the dc motor. This way, there will be no forward and backward oscillations in the motor and it will only turn in one direction.

Is my approach right?
 

rjenkinsgb

Well-Known Member
Most Helpful Member
This is the basis for a suitable circuit:

It has a rectifier, a comparator to decide if the voltage from the rectifier is high enough to mean it should switch, then a transistor to switch the relay on or off.

If you motor you have is low enough current you could use a higher current rated transistor to switch power to the motor directly - though that means it can only ever turn one way; it would need a spring to reset the lock.

Using the relay plus switches in the motor circuit to detect the ends of travel of the lock mechanism means it could reverse and lock again when the relay releases; that's a separate bit of circuitry from the remote switch itself.

Components are not critical, any small signal diodes for the input rectifier and there are hundreds of different opamps that would work in place of the 3140, just about any single-supply opamp that runs at whatever supply voltage you want to use.


I'd just build the rectifier part first, as far as R1 and see what voltage you get across C2.
Rx is just a direct connection - the circuit is intended to be able to take high voltages in to it, which you are not using.

The 5.6V zener is likely too high, which is why you need to find out what voltage you get from the rectifier when the phone rings. Then set the threshold voltage (that was 5.6) to somewhere between half and two thirds the voltage you measure. You may only need to use one or two signal diodes to give a low switching point.
 

Mickster

Well-Known Member
Most Helpful Member
Does the phone have a vibration motor?
 

Asheekay

New Member
Does the phone have a vibration motor?
Yes it does.

Oh ... do you mean that I should utilize the purely DC current of the vibration motor instead of AC feed going to the speakers??

Ahan! That means removing the rectifier part altogether and just amplifying the vibration feed and then connected the amplified output to the lock motor should work?

rjenkinsgb said:
If you motor you have is low enough current you could use a higher current rated transistor to switch power to the motor directly - though that means it can only ever turn one way; it would need a spring to reset the lock.
It is 1 watt motor. So being 12 volts means it should operate at 1/12 amps.

That's why I am thinking maybe my initial idea (using a transistor as a switch to turn on an external circuit, as I posted in my opening post) wasn't so bad after all. Except that I need to rectify the AC feed of the ringing signal to DC first.
 

shortbus=

Well-Known Member
Out of the 3 cupboards I use in my home, there's one that also holds items sometimes needed by others.

This cupboard, I would like to be able to unlock from afar, without using manual keys, of course.

After thinking and trying a few options including:
One you haven't yet mentioned and is also the easiest, and least expensive. Just give the people you trust to get in the cupboard a key for a manual non-smart lock that is on that cupboard.
 

crutschow

Well-Known Member
Most Helpful Member
That means removing the rectifier part altogether and just amplifying the vibration feed and then connected the amplified output to the lock motor should work?
Yes, it should.
Can you measure the voltage to the vibration motor?
 

alec_t

Well-Known Member
Most Helpful Member
1) How will you re-lock the cupboard?
2) How will you prevent a junk call or wrong-number call from unlocking the cupboard?
 

Asheekay

New Member
1) How will you re-lock the cupboard?
2) How will you prevent a junk call or wrong-number call from unlocking the cupboard?
1- It is auto / spring locked. Closing the door locks it.

2- By using custom ringtones. All numbers (except the authorised ones) are assigned a completely silent tone. Authorised numbers have a loud ring.
 

narkeleptk

Active Member
1- It is auto / spring locked. Closing the door locks it.
You will need to consider, the person could just hold the door open.
If its something that is ok to access by the person at home your sending the unlock code to, then there is really no reason to just not install a normal lock and give that person a key?

Its a fun project and all I guess but not very functional, and not very secure at that.
Those latch style locks are easily bypassed with a thin plastic shim. The problem with bolt latches tho is the door needs to be shut before it can lock.
 

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