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I want to connect a Chamberlain 942FP / 379LM fingerprint reader garage door "keypad" to a Chamberlain B750T (I think a.k.a. B750) opener. Unfortunately the fingerprint reader is much older than the opener, so I couldn't pair it with the opener wirelessly.

Fortunately however, the fingerprint reader comes with a receiver which plugs into an AC outlet and can be connected with 2 wires to the opener. This receiver also has a push button to allow for easy testing without having to know if the fingerprint aspect is working, and when pushing it, I could hear a relay click, but unfortunately it doesn't appear to be compatible with the opener as nothing happens.

I hooked up a multimeter to the wires I connected to the receiver unit and can see a voltage change when pressing the button (appears to be AC). The instructions for the receiver unit say it doesn't matter what wire is hooked up to where on the opener (i.e. red vs white), but the push buttons that came with the opener unit have terminals specifically labeled white and red, and the inputs to the opener have color coded wire hookups. Also the push buttons that came with it do not plug into AC power, so I'm assuming they draw power from the opener itself through the low voltage bell wire.

I suspect the opener needs some DC signal whereas the receiver that goes with the fingerprint reader gives out an AC signal. Any ideas on how to make them compatible (buying something is fine if needed).

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  • At the opener end, I presume you have the wires connected red-red and white-white. Have you tried swapping them at the transmitter end? While the transmitter doesn't seem to care, the receiver might.
    – FreeMan
    Mar 2 at 14:59
  • Do you have the manual for the fingerprint reader, have you followed all of its installation steps? Do the LEDs on the transmitter and receiver indicate what they should? Have you followed the manual's troubleshooting steps? Which ones, and what trouble was found?
    – jay613
    Mar 3 at 23:24

4 Answers 4

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Garage door opener remotes typically operate by shorting the wire. You can replace the garage door button (on the wall near the door to the house) with a simple doorbell since that is all it needs to make contact. Some buttons have extra functions like locking or turning on the light, these are done with resistors or other signal modulation on the wires.

Newer garage doors can pass additional functionality to the button panel, I had one that was Wifi and MyQ compatible so it obviously did a little more than just short the wires together. But on the other hand, many people are making their very old garage door openers "smart" by just adding a wifi-controlled relay to short the wires, and tucking the "smart device" up above the opener.

For your situation, use a multimeter to see if there's any resistance between the two terminal screws on the Receiver as it clicks while you test it. It may be hard to tell, because like a doorbell button, contact is only made for a moment (this is all it takes to ring a doorbell, and all it takes to command the garage door to move). You should see a brief "0 ohms" as the click happens. The AC power to the Receiver is probably only to handle the communication with the fingerprint box.

As far as permanently wiring it in: you need it matched with the red and white terminals that the wall button is already connected to. You can either use small wire nuts (1 for the white wires, 1 for the red wires) or try to fit 2 wires into the red and white terminals. It doesn't matter which terminal of the Receiver connects to which terminal on the opener, match color coding with the button wire to keep things simple.

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    I tested the resistance and it changed when the receiver was activated. To get everything working I ended up connecting the two terminals of the receiver to two of the button terminals of an extra remote control I had. Now when the receiver gets a signal it activates the remote which in turn talks to the garage door and all is good.
    – g491
    Mar 8 at 0:12
  • This reenforces my view that the wires from the receiver, inserted into the motor's push connectors, were not making contact. Note your fingerprint sensor now relies on two batteries.
    – jay613
    Mar 8 at 13:02
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Some time ago... long ago... I did some work on a home automation system which involved interfacing to a Chamberlain/Lift Master door opener. The model number has faded from my memory but I do recall discovering that the operator unit had the capability of using a serial communication with the wired wall-mounted controller unit. (That could look like an AC voltage on a volt meter, by the way.)

Back in the Olden Days™ door operators were controlled by a simple switch which shorted the wires together. I recall there was something quirky about the way wired buttons worked with that operator I played with. It could be controlled by a simple short-to-operate wall switch, or it could be controlled with the fancy serial communication controller unit. Chaos, or rather failure to operate, resulted when we attempted to use both styles of controls.

I suggest that you disconnect all the controls from the door operator and then reboot it. Then try operating the door by simply shorting the control input terminals with a jumper wire. The disconnect power from the operator, connect your fingerprint receiver thing to the operator, and power the operator up again. Test for operation with the fingerprint reader or its built-in test button.

Hopefully that sequence of testing will reveal whether your operator works with each of the different classes of controls in isolation. If it does work with the fingerprint device, it might also support the fingerprint together with a same-generation wall control (ie one that's older than the wall control you have now). Or use the wired input for the fingerprint device and replace the wired wall control with a wireless remote mounted to the wall.

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To get everything working I ended up connecting the two terminals of the receiver to two of the button terminals of an extra remote control I had. Now when the receiver gets a signal it activates the remote which in turn talks to the garage door and all is good. Here's a pic of it in working prototype (pre-solder).

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The terminals on the receiver do not produce any voltage, neither AC nor DC. Using a voltmeter on them will produce unpredictable results. Measure resistance, as described in the other answer.

Ensure the transmitter and receiver are correctly installed and functioning, and their indicator lights indicate success at every step. If you don't get that far, use the manual to troubleshoot.

If you do get that far, the most likely problem is that those push-in connectors on the motor are fickle. It's hard to know when a wire is properly pushed in and making contact, and MUCH harder when you shove two wires in. If a slight tug on a wire removes it, it wasn't inserted properly. Make sure wires aren't inserted so far that the insulation is interfering with the contact. With two wires strip the wires back a little more and firmly twist them together before inserting in the connector.

Also make sure the red wire is inserted into the motor's red "button" terminal and not the one for the safety eye.

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