For some time I wanted to build a remote switch, main purpose – for firework ignition (or similar tasks). However, I wanted to create a RF remote without the use of microcontrollers, if possible to modify an existing device to do such a task.
Only one channel required (on/off state). Receiver device, after receiving a signal from the remote, should be able to stay on for a few seconds. Minimum distance required is 20m in the open space.
While searching over the internet, and came across a forum site, (http://www.elitesecurity.org/t373470 (forum is on the Serbian)) where I found schematics for hacking a doorbell, so I wanted to try it out. This setup seemed to me simple and easy to do. Some changes to the schematics are needed for this circuit to be fully functional.
Caution required when device is used in densely populated areas. Depending on the model of the wireless bell, it has a working frequency of 315 MHz or 433 MHz, it is certainly possible that other device that operates on the same frequency could trigger the device (Because of this, I used this device for firework ignition outdoors away from houses). As it lacks some safety issues the greatest use of it would be to use it to (harmlessly) prank your friends
Again, you are responsible for yourself and your actions.
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Step 1: Materials and Tools Needed
1x cheap Chinese wireless doorbell (with transmitter and receiver included)
1x diode 1n4001
1x 100µF electrolytic capacitor
1x 470 Ω resistor
1x BD681 darlington transistor
1x relay 5v
1x old 9 volt battery (or old 9v battery clip)
1x new 9v battery
Heat shrink tube or insulating tape
Batteries for receiver - depends on the model
Multimeter – optional
First open the receiver, unscrew any screws and carefully open case. Batteries and receiver are connected to one part of the case; however the speaker is probably connected to the other part. Be careful not to tear any wire.
Connect, and solder negative terminal of the receiver (ground) with wire (which will be connected to the negative terminal of 9v battery, later).
Cut the wires that are connected to the speaker. Speaker is no longer needed; it can be removed from the case.
Following schematics connect LED across free speaker terminals. A multimeter can be used to find a positive terminal, although I found that polarity changes over both terminals so, at this point do not worry about polarity.
Next, connect a diode (mine was 1n4001) to the one of the terminals (see schematics).
Time to place a 100µF electrolytic capacitor. Just watch polarity! Without the capacitor, finished device will create relay that ‘sing’ doorbell melodies, and be barely useful.
The capacitor role here is to store some charge received from the speaker terminals, and slowly discharge through darlington transistor, creating more smoother performance.
At least 10µF is needed to smooth signal received from speaker terminals, however upper limit don’t exist, so you could hook a 1000µF or higher value capacitor and it will work fine, and it will extend the ON state of the device, after a signal from speaker disappear.
Connect 470 Ω resistor, and base of BD681 darlington transistor. Connect the emitter to the negative (ground) wire. Watch the pin-out of the transistor. Download data-sheet if needed.
(I’m not sure if regular npn transistor can be used in this setup.)
Now solder a wire to collector of the BD681, and the other end of wire solder to one of the 5v terminals of the relay. *Decide how much wire length you needed.
Solder the second wire to the relay.
Now a 9volt battery clip is needed. It can be made from an old 9v battery; instructions for this can be found here: https://www.instructables.com/id/quick-cheap-9V-ba...
Now solder a free wire from the relay to the positive terminal of battery clip, and negative terminal to the wire that is connected to the negative (ground) of the receiver.
Insulate terminals with tape or shrink tube, close box, put in the batteries.
And it’s done.
Step 4: Testing
For first test connect 9v battery, make some distance press the remote and with LED turned ON relay should switch (you should hear the sound).
After pressing a remote button, relay switches ON for about 3 -10 seconds (depending on the bell melody). Device cannot work continuously, but it is well suited for use as a very simple firework ignition (and many more).
Thanks for viewing!
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