Turning a Wireless Doorbell Into a Remote Controlled Relay.
BUILD TIME: 2 hours
DIFFICULTY RATING: Intermediate
Many people use a wireless doorbell in their home or workplace, with a battery powered radio transmitter triggered by a button that can be placed at the door or reception desk, and a receiver connected to a sound-making circuit and amplifier at the other end. This can be placed where it is likely to be heard by anyone inside the home, or out in the workshop, or whatever the case may be. Many have different sounds that can be made, and most have volume controls as well. Some have an LED visual indicator, although not all of them do.
As salespeople at the retail stores where these devices are sold will confirm, there are quite often people who want to make them louder, despite the volume some units are inherently capable of. These requests often come from those in high-noise environments, or who have hearing loss. We had one of these units lying around at the office. No one can remember why we acquired it, so we decided to have some fun. The discussion quickly turned from making it louder to give it a visual output that may solve the background-noise problem. What better way to do this, than with a relay that can drive whatever load you want it to? All of a sudden, we have a general purpose wirelessly-controlled relay.
We started by dismantling the receiver unit. Sadly there isn’t much in it. Two switches for power and volume, two capacitors, a resistor, an inductor, jumpers, an LED, and two epoxy-blob circuit boards. These are where the main business takes place, and with no hope of tapping into them or even analysing them, we reassembled the unit. Instead, we debated taking the trigger for our relay from the LED. This is mounted to protrude through the front panel, and flashes at around 2 Hz. The problem is, not all wireless doorbells have this feature, and we wanted our design to be universally applicable.
This left the speaker output as our trigger. What was needed was a circuit that could tap the AC signal to the speaker, without placing undue load on the internal amplifier, and drive a transistor output. We decided to do this with a coupling capacitor, and have the output rectified so that DC can be fed into another capacitor and the input of an Op-amp. The signal coming through the rectifier will vary in intensity with the volume of the sound, such as the on/off of the cuckoo sound or chimes, or the sharp ‘ding’ and gradual decay of the bell sound. The second capacitor will help smooth the pulses of DC relating to the varied volume in the different sounds. It will also slowly discharge after the signal ceases to provide a longer ‘on’ time, which warrants a resistor here to slow the discharge. The Op-amp will be configured as a comparator, and hysteresis will be added to give a longer ‘on’ time by separating the ‘on’ and ‘off’ thresholds.
The output stage will be a transistor operating a relay. This will have its own flyback diode.
The whole circuit will operate from its own power supply. Again, this decision is based on the need to make the circuit universal. While our doorbell operates from three AA batteries, giving 4.5V, others operate from two batteries totalling 3V. That’s a best-case scenario, too, as the voltage will fall as the batteries are used. While there are some relays around with 5V coils that will operate from 4.5V, we felt 3V was pushing our (and makers’) luck. This is unfortunate, as having an all-in-one design with no external power would be nice, but at least you’ll likely need separate power anyway for whatever load you choose to drive with the relay.
Of course, you may have different circumstances and needs, and may make your own modified design decisions. That’s why we’ve explained so much just now! You can assess the design aspects of the project against our thinking, and decide what you can change based on your own needs.
We’re basing our design on the venerable LM358. Although many Op-amps can be made to run from a single supply, this Op-amp IC is designed to. It is also common and cheap, already existing in the parts boxes of many makers.
The rest of the circuit is as we proposed earlier. The resistors to slowly discharge the capacitor are a 100kΩ fixed resistor and a 100kΩ potentiometer, wired as a rheostat. This gives a healthy minimum to the discharge, while allowing the delay to be extended reasonably.
The whole assembly is built on the PCB equivalent of the solderless breadboards we’re familiar with, which are available at many electronics retailers (See parts list).
Start by checking over your PCB for manufacturing errors, then mount the LM358. Best practice says to mount low-profile components like wire links and resistors first, but on this board, you have to count holes for accurate placement. In this case, mounting the IC first makes it much easier to count from this, than counting from the board edge. Alternatively, you could use an IC socket and insert the IC at the end of the build.
|1 x Wireless Doorbell#
|1 x Pack of Breadboard Wire Links
|1m x Light Duty Speaker Wire
|1 x 2kΩ Resistor*
|1 x 5.6kΩ Resistor*
|1 x 9.1kΩ Resistor*
|1 x 56kΩ Resistor*
|1 x 100kΩ Resistor*
|1 x 1MΩ Resistor*
|1 x 100kΩ Potentiometer
|1 x 100nF Capacitor
|1 x 10μF Electrolytic Capacitor
|2 x 1N4148 Diodes*
|1 x 1N4004 Diode*
|1 x BC337 NPN Transistor
|1 x LM358 IC
|4 x PCB Pins*
|4 x PCB Pin Sockets*
|1 x Two-way Terminal Block
|1 x Relay of Choice
|1 x Prototyping Breadboard
|1 x Cradle Relay
|1 x Cradle Relay Base
|1 x Reversing Buzzer
|1 x Strobe
* Quantity required, may only be sold in packs.
# The unit we used was discontinued. Part numbers listed are for current lines, but we have not tested them.
^ Not identical
Install the wire links, resistors, and diodes. Note that the wire links we used are standard breadboard wire links, and are coloured accordingly. We found it easier to colour the power rails with a coloured marker first.
Solder all the currently-mounted components and trim the leads, before adding the capacitors, transistor, and PCB pins. In this type of board, the PCB pins usually friction-fit which helps keep them in place when you turn the board over to solder. This is a pleasant contrast to custom PCBs, from which PCB pins usually drop out.
Finally, add the potentiometer and terminal block.
Open up your wireless doorbell, and locate its speaker. Ours was at the end of short wires, and mounted to the inside of the lid. Solder a length of speaker wire to the speaker terminals, in addition to the wiring already connected to them. File a small slot in the side of the case for this wire to exit, and reassemble the doorbell. Crimp and solder PCB pin sockets to the other end of this wire, and heatshrink the exposed metal.
DO NOT ATTEMPT TO USE THIS PROJECT WITH MAINS VOLTAGES OR LOADS.
Wiring the relay depends on which relay you choose. We prototyped with a spare PCB relay, so we soldered wires on to its coil and connected them to the terminal block. If your relay has a screw-terminal or spade connector base, as our final design did, connect wires as relevant. The same goes for the switched load: Its wiring will be specific to your circumstances.
We designed the circuit to drive a relay so that you can switch whatever you want. If you are trying to use the doorbell in a high-noise environment, you could follow our example and connect a reversing buzzer and flashing light. For this, we swapped the PCB relay for a DPDT cradle type with a screw terminal base. We used the ‘common’ terminals to wire parallel to the power supply for the circuit, and the ‘normally open’ contacts to connect the load.
Other than that, the connected load is only limited by your imagination, your power supply, and the relay rating. You could switch on a 12V LED light strip up your driveway from your car or garden lights.
It may switch a fan. You could easily connect a second circuit, a latching circuit, to switch alternately on and off based on the shorter ‘on’ times of the relay in this circuit.
WHERE TO FROM HERE?
The first thing you may want to do is remove the annoying sound. This could be relevant to people who want to use this as a remote controlled relay and not as a supersized doorbell. To absolutely guarantee circuit functionality, it’s best to replace the speaker with a 1W 8Ω resistor to maintain amplifier load. Everything else remains the same.
If you did want a single power supply for your project, you could build a regulator to match your unit’s battery voltage and connect it to your project’s supply. This way, a 12V battery could run your circuit and relay, the doorbell, and the load.