# Keypad for door lock + led light?

#### DimiLa

Joined Jun 8, 2019
7
Is it possible to connect an led in parallel with a lock mechanism?

I am working on a project. I have a keypad panel that unlocks a door and I want to add in the circuit an led indicator(when the lock is released I want the led to turn on. When connecting the lock with the led in parallel the keypad does not work properly. The lock uses 1amp and the led needs less than 350ma. Would a resistor on the led solve the problem? If yes, what resistor should I use? (it's obvious I don't know much about electronics but I wish to learn...). Any ideas or ideas or advice is most welcome! Thanks community!

#### crutschow

Joined Mar 14, 2008
25,269
350mA is a large current for an LED. Most indicator LEDs take less than 20mA.
Bare LEDs always require a resistor in series to limit the current, as they are current operated devices.

What is the voltage across the lock, and what is the LED part?

#### Dodgydave

Joined Jun 22, 2012
9,317
Yes, use a 470 ohms to 1K resistor in series with the led across the lock output.

#### MisterBill2

Joined Jan 23, 2018
6,750
All of the electric door latches that I am familiar with run on 24 volts, while most of the LEDs run on between 1.8 volts and 3 volts. So indeed the LED MUST have a series resistor to limit the current.

#### DimiLa

Joined Jun 8, 2019
7
350mA is a large current for an LED. Most indicator LEDs take less than 20mA.
Bare LEDs always require a resistor in series to limit the current, as they are current operated devices.

What is the voltage across the lock, and what is the LED part?
Yes, use a 470 ohms to 1K resistor in series with the led across the lock output.
Thank you so much for your reply. I'll give it a try as soon as I get the parts and let you know.

#### DimiLa

Joined Jun 8, 2019
7
350mA is a large current for an LED. Most indicator LEDs take less than 20mA.
Bare LEDs always require a resistor in series to limit the current, as they are current operated devices.

What is the voltage across the lock, and what is the LED part?
Both run with 12v. The lock's sticker says 1Amp and the LED's says up to 350mA.

#### DimiLa

Joined Jun 8, 2019
7
Both run with 12v. The lock's sticker says 1Amp and the LED's says up to 350mA.
It's not a bare led, it's sold as a closet light that you would normally install as is connected to a DC 12V transformer of 350mA.

#### MisterBill2

Joined Jan 23, 2018
6,750
OK, now the situation is much clearer. The LED device is not a simple LED. That makes a very big difference. once again a bunch of details are now being provided that were not given along with the initial question. So now it is possible to provide some advice related to the actual problem. The first concern is the polarity of the voltage for that LED light assembly. If it is intended to have DC power it is probably polarity sensitive and will only work with the correct polarity applied. In addition, the electric lock solenoid may have a suppression diode installed, which will make it also polarity sensitive. If either of those is connected in reversed polarity it will not work. Next is the question of the current rating of that power supply. What is the current rating? It may not be adequate to power the lock, the light, and the keypad circuits. So there you have three possible sources of the problem. In addition, it is possible that the output current rating of the keypad assembly is not adequate for the load. It is also possible that the keypad output to unlock is only a short pulse.

#### DimiLa

Joined Jun 8, 2019
7
OK, now the situation is much clearer. The LED device is not a simple LED. That makes a very big difference. once again a bunch of details are now being provided that were not given along with the initial question. So now it is possible to provide some advice related to the actual problem. The first concern is the polarity of the voltage for that LED light assembly. If it is intended to have DC power it is probably polarity sensitive and will only work with the correct polarity applied. In addition, the electric lock solenoid may have a suppression diode installed, which will make it also polarity sensitive. If either of those is connected in reversed polarity it will not work. Next is the question of the current rating of that power supply. What is the current rating? It may not be adequate to power the lock, the light, and the keypad circuits. So there you have three possible sources of the problem. In addition, it is possible that the output current rating of the keypad assembly is not adequate for the load. It is also possible that the keypad output to unlock is only a short pulse.
Well, first of all the lock is not polarity sensitive. On the other hand the light is polarity sensitive. Also, the power supply is 12VDC, 2Amp. In addition, the keypad, if I am not mistaken, provides constant current for the lock as it uses a magnet to hold the lock open (usually used as a drawer locking mechanism). You can program the lock to hold it open from 1 to 99 seconds. The output of the keypad, the as per my measurement, is 12V.

#### DimiLa

Joined Jun 8, 2019
7

#### DimiLa

Joined Jun 8, 2019
7
I tried it and it worked as a charm! Thank you all for your precious time!

#### MisterBill2

Joined Jan 23, 2018
6,750
OK, it is clear that the lock does not have an internal diode like some lock solenoids have. That is good. And the voltage out from the keypad is 12 volts, as it should be. But what is the voltage from the keypad with both the lock and the LED light connected? And another question is about the intended power source for the light: Is it intended to work with any 12 volt source? Or it it intended to work with a very specific power source that is normally sold with it? Can you try lighting the LED from that power supply directly, without it being controlled by the keypad? That will verify that it is working correctly. That is the next logical step in discovering what the problem is.

Joined Feb 20, 2016
3,597
You ALWAYS need resistors with LEDs. Unless the supply is a constant current one with the correct current set.
Why do you need such a bright light for the lock indicator?
Are you wanting to illuminate the doorway?

The power supply probably will provide constant VOLTAGE, not constant current. There is a big difference in power supply terminology.

It's not a bare led, it's sold as a closet light that you would normally install as is connected to a DC 12V transformer of 350mA.
Is there any more to the LED than in the picture, like stuff on the back of the heatsink or parts on the end of the wire?
That looks like it is just an LED so a current control mechanism will be needed. LEDs are current operated devices, not voltage. Usually, they have a more or less constant forward voltage drop of around 2 volts or so, then if you put it on a 12V supply without a resistor or some other current control, you will instantly pop it.
Are you after an indicator or illumination?
A suitable resistor will be in the area of 39 Ohms and at least rated at 5 Watts it you are after 300mA or so. But that will be pretty bright!
A normal 3.5mm or 5mm LED as an indicator with a 1K resistor, as mentioned above, will give you a good lock operated pilot.