My dad approached me with an interesting problem to solve ... the doorbell in their house is hard to hear from the tv room or dining room. Given that 90% of the deliveries made to the house are stuff I've ordered, I had vested interest to solve this.
The usual solution to this is to buy one of the wireless doorbells that has 1 switch and multiple bell units -- you just stick the switch to your gate, and place the bell units wherever you like. However there were some good reasons against this method:
- There's a distance of 40 feet from the house, and being on different levels, there's concrete and earth in between. And that is just measuring to the front of the house ... I had doubts on the wireless signal ability to reach into the kitchen and tv room which are to the back of the house
- The existing doorbell has a wire running all the way to the gate. I'd like to make use of this wire.
- The existing doorbell is quite a decent unit, so it would be ideal to keep it in use and supplement it with additional bells.
- The gate has a doorbell button built into it ... it would be ugly to have a stick-on replacement wireless button.
- Indicative of a lack of confidence in my skills, my dad asked for a "bypass switch" so that things could be reverted back exactly to how they were, should something stop working
Now this is probably something easily solved by anyone with even basic skills in electronics, but given that I've been dabbling in this stuff only recently, it turned out into quite an interesting learning project. Also, most of the stuff I've been working on has had a CPU as a controller, and therefore are mostly software driven. This one would need have its logic driven by the circuit itself (#NoCPU !!)
I started off by purchasing a wireless doorbell, a Qusun brand that seems to be the standard stocked everywhere. This came with a button transmitter unit (powered by a "A23" 12V battery) and two receiver bell units (one for the kitchen, and one for the tv room -- the existing bell can serve the rest of the house). I went for the AC powered bell units as I figured these would require less maintenance. The bell can be selected from a choice of 36 different tunes -- I chose the standard "ding-dong".
The first problem to solve was how to make the wireless bell ring without physically pushing the doorbell button. I opened up the button transmitter, and found the points on the circuit board where the push button switch made contact, and soldered two wires that lead out of the case. I tested it by shorting the wires, and the wireless bell rang.
simulating a button press
The next thing was to figure out how the old doorbell worked. I opened the unit and it turns out to be a model 3230 made by Friedland well over 20 years ago (when stuff was Quality Stuff). I even managed to find a manual online.
Knowing where the AC wires were to keep well away from, I found the two terminals which had black wires connected to them. I figured these wires ran to the gate. Tested it by shorting the two points on the unit, and the doorbell rang. Ok we have control!
So what I needed next is the ability to close two existing circuits (the wired doorbell switch circuit and also the wireless doorbell switch circuit) when the button at the gate was pressed.
I figured a Dual Pole Dual Throw Relay would be a good solution for this (This video explains how a relay works). A "Dual Pole Dual Throw" relay just means that the one solenoid circuit can be used to drive two other circuits. Using a relay also has the advantage of isolating the doorbell at the gate (with it's long run of surge attracting wire) from the wireless doorbell transmitter circuitry, which to be fair was probably not designed to be connected in that way).
I did some experiments with a relay and it seemed to work the way I needed it to. The only thing is that I would need a DC power source to drive the solenoid. So I would need a battery pack of some sort.
The real tricky part was figuring out how to implement a bypass switch. I went with a dual pole dual throw switch, wired as follows:
With the switch in the top position, the push button is connected directly to the old wired doorbell, i.e. it behaves exactly like how it used to. With the switch in the down position, pressing the push button causes the solenoid in the relay to activate, which causes the wired doorbell switch and also the wireless doorbell switch to close.
Since theoretically everything seems in order, the next part is the part I hate most ... building the circuit. I used a piece of perfboard and soldered on three terminal blocks for external connections. I also soldered on 6 header pins to connect the DPDT switch to, and also soldered on the relay (in hindsight, maybe I should have soldered an IC socket to the board and then plugged the relay into it? It would make the relay easy to swap out if the solenoid burnt out). Then came joining all the bits together to form the circuits, mostly achieved with some wire runs and solder bridges.
Bottom of the board. Please excuse the ugly work, it's my first attempt!
Top of the board
Actually it took me three tries to get it right ... the first time I soldered the terminal blocks on backwards! The second unit worked fine until I tried to measure the current flow, and discovered that there was a 3-4 mA drain from the battery even when the switch was not being pressed. This would drain the battery over time. I couldn't find the source and ended up rebuilding the entire board, only to later discover that there was a small short at the bypass toggle switch!
In an earlier version I had crimped DuPont connectors to connect the bypass toggle switch wire to the circuit board ... but these turned out to be problematic (I believe my wires were too thick to use with DuPont connectors). In the end i just soldered the wires directly to the pins and covered it it with hot-melt glue to keep them still.
I used a pack of 6x1.5V batteries rather than a single 9V battery as they have more capacity and cost less. Actually given that it only uses the battery power for the few seconds the doorbell button is pressed I think the batteries will die out from age first.
Next was installation day! It was rainy but I decided to risk it anyway ... climbed up a ladder to where the doorbell was located, disconnected the gate push-button wires from the existing doorbell, and installed my new "1 Switch, 2 Doorbells" unit.
Redirecting the doorbell push button wires
"1 Switch, 2 Doorbells" unit
Tested it ... needed an umbrella to to get to the gate in the pouring rain ... but it was worth it, everything worked!
So far it has been running for nearly a week and has survived a number of thunderstorms, so it looks like my first practical project has been a success!