July 15, 2012 Subscribe Where might I find a replacement (low-profile) doorbell transformer that is similar to this? It is about the size of a Matchbox or Hot Wheels toy car and has FRONT, REAR and TRANS connectors. The replacement transformer I bought looks like this, which is about the size of a tennis ball and has white, black and green wires, but no FRONT, REAR and TRANS connectors. Big chain hardware stores shrug their shoulders, lighting stores only carry the big TF I linked to, so where might someone go to find something like this, to fix an older doorbell?Check out the new version of this, now with bell enable/disable function!A lonely doorbell activated by a boring push-button at the front door is not very exciting in a world of Arduino and “Internet of Things”. I’ve been wanting to Arduinize the doorbell into the Moteino Framework gateway interface so that I could:Below is a schematic of what a typical wired door bell circuit looks like, and also a photo of what it looks like in my house.
The dotted green rectangle is the circuit that I have physical access to – pictured next to it in my basement (and I don’t have a back door button):I have a single button (front door) and probably most houses do. So I only care to inject my solution for that front door button. There is typically a 16VAC transformer powered from mains that activates the chime when the switch is pressed. So there are a few issues to solve in order to tap into this circuit, detect button presses and also being able to control it via Moteino:I will explain these points step by step. Let’s get to work!1. Getting 5V power from a 16VAC sourceThis is pretty trivial. We need to tap directly to the 2 terminals of the transformer, this is the 16V AC source that we feed from. All we need is a rectifier diode (1N4004 or 1N4007 will do) + smoothing capacitor (100uF is great, rated at least 25V or more) to go from a full sine wave to a half wave to a low ripple DC outout. My doorbell transformer is 16VAC rated and the peak-to-peak output is somewhere around 40Vpp.
We then get a DC output that is around 18V which is too much for the Moteino so we can add a MC7805 5V regulator (which accepts up to 35V input) to get a stable and safe 5V source for the low power electronics. Here’s a schematic that shows how to go from AC to DC with a single rectifier diode and a smoothing capacitor, I tried all of this on a breadboard with a 12VAC transformer but it works the same as the 16VAC one:Here is what the output on a scope looks like, straight from the rectifier circuit. Adding a the 7805 regulator brings it down to a smooth 5V, just what we’re after: 2. Detecting AC current presence/flow with digital logicOnce we get the needed 5V DC power, we need to monitor the door bell button and determine when it is pressed. To do this we tap into the third tap – this is where the transformer output is wired to the door bell *through the front door push button*. When the button is pressed, current flows to the bell and the chime solenoid energizes and plays the high note, and when released the solenoid comes to rest and hits the low note.
So when the front button is pressed, this third tap (often exposed like in my case) will have flow from the other side of the switch – the transformer.To detect AC flow we can use a nifty little IC that can detect AC zero-crossing – the H11AA1 optocoupler. I found this video which explains it pretty well and has a schematic. The IC is optocoupled internally so it’s fully isolated and can be used to detect live mains AC zero crossing as well as lower AC like our 16VAC transformer output, it works the same. It basically outputs a short DC HIGH pulse on the DC side when the AC is crossing “zero”. Here’s the output from the IC on my scope – the blue is the 16VAC and yellow is the pulsed output from the H11AA1 – note the blue is 60hz and yellow is 120hz:To output a constant DC HIGH pulse instead of 120hz pulses, we can do a little trick. Using a single PNP transistor we can invert the logic and we’re almost there. Here’s an article that describes how to invert digital logic with transistors and a diagram excerpt from that article that makes it plain to see what we’re doing.
Also the inverted output (compare with previous scope shot):To smooth it out to DC we use the same principle as in the first part – add a smoothing capacitor that will delay the voltage drop enough to keep it at constant “DC” voltage levels. Here is the breadboard with the PNP transistor, 10uF capacitor (any will do) and a LED for visual indication – this LED input will be shared with the Moteino digital pin that monitors this AC presence.3. Virtually triggering the door bell via MoteinoAgain, quite trivial. The button is just a momentary pushbutton. We can add a small relay in parallel and power it from 2 digital pins like the GarageMote relay.Here is the complete schematic of this whole circuit. I tried this in DigiKey’s SchemeIt, a polished but otherwise pretty clumsy to use schematic builder:Here’s the strip board implementation mess-up. I added a green “power” LED on the 16V rectified output, it should always be ON when the circuit is plugged in.As I mentioned before, there are only 3 connections/taps into the existing circuit.
The 2 red/black wires get the power from the 16VAC transformer. The yellow wire is the pushbutton tap which is used for the H11AA1 detector circuit and also on the relay output to simulate a door bell button press.The Moteino sketch for this is published here. Here’s how this shows up in the gateway interface, and the graph showing me playing around with the door bell at different times in the afternoon:If you have suggestions to improve the circuit or make it easier to understand add your comments below.AVAILABLE FOR PAY & PICK UP AVAILABLE FOR IN-STORE PURCHASE ONLY THIS PRODUCT IS DISCONTINUED Door chime transformer16 volt 10VAIdeal for single chime door bellsChrome finishDimensions: 2 x 2¾" (5 x 6.9 cm) Here's what others had to say... Date - Newest First Date - Oldest First Rating - High to Low Rating - Low to High Helpfulness - High to Low Helpfulness - Low to HighEdit ArticleHow to Repair a Door Chime If your door chime no longer provides the familiar tones and merely hums or buzzes, you can probably fix it without too much trouble.
Try this before replacing it. Remove the cover of the chime unit. Look for obviously broken or missing parts. Tighten electrical terminal screws. Listen carefully and then observe the moving parts while an assistant presses the doorbell button several times. Check and tighten the electrical points (fuses and circuit breakers, terminal screws of the doorbell button(s), and transformer) if unable to hear or see the indications above and try again. Check the plunger for freedom of movement. Clean the moving parts of the chime unit. Cleaning can be done in place or removed from the wall. Spray a short blast of electrical contact cleaner at the moving parts. Once the plunger is able to move freely, the chime unit is ready to be reconnected and tested by pressing the doorbell button. Compare the voltage rating of the chime (likely 12 to 24 volts) with the rated voltage output stamped on the transformer. Remove wires from doorbell buttons and touch wires together.