# Beginner in electronics, simple question about a mechanical relay circuit

#### gerases

Joined Oct 29, 2012
186
Hello, I'm trying to build a circuit below where the LED will toggle every second. I'm using this relay: https://www.farnell.com/datasheets/1918250.pdf. -- model v23105-a5401-a201. So I am at a point where I don't even have the LED+resistor in the circuit. I just want the relay to turn-on-off every second. I have a 1000uF capacitor. What happens is that with just the relay and the power supply connected, the relay thrashes, which I expect. When I connect the capacitor, things slow down but I want to slow them down to once per second. I don't have a higher value capacitor, so I was trying to put a resistor (R2) in. Above about 60 Ohm, the relay stops working completely though. My power supply is 5V but I've also tried 9V.

Here's a portion of the datasheet:

Does that mean the coil must have 5V of voltage minimum to work? If so, it would explain why putting a resistor in makes it stop working -- since there will be a voltage drop on the resistor?

For simplicity, according to my calculations, with a 1000uF capacitor I would need a 200 Ohm resistor to get the time constant of 0.2 second. Since it takes 5 such constants to charge a capacitor fully, I will get my 1-second cycles. I might be off, but it's close. The problem however is that this relay stops working with much smaller value resistors.

Any ideas are welcome!

Thanks!!

#### LowQCab

Joined Nov 6, 2012
3,937
Remove the Resistor, it only confuses the problem.

Your Time-Constant calculations should use only
the Capacitance, and the DC-Resistance of the Relay-Coil.
That's the whole thing.

Do You expect a "Pulse", or a "50-50" Duty-Cycle ?
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#### sghioto

Joined Dec 31, 2017
5,369
The relay you chose has a low coil resistance of only 62 ohms. Any resistor in series with it will drop the voltage. The specs indicate that relay requires a minimum of 3.5 volts to operate. With a 200 ohm resistor you would need a 15 volt supply.

#### gerases

Joined Oct 29, 2012
186
Do You expect a "Pulse", or a "50-50" Duty-Cycle ?
Hmm, what's the difference ?

#### eetech00

Joined Jun 8, 2013
3,820
Try 3 2200uF (6600uF) capacitors in parallel, without R2, and with 5v supply.

#### gerases

Joined Oct 29, 2012
186
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#### LowQCab

Joined Nov 6, 2012
3,937
If You would state exactly what it is that You are trying to accomplish,
there may be far superior methods to achieve that end.

Are You trying to learn about Electronics, or, do You just want a flashing LED ?
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#### gerases

Joined Oct 29, 2012
186
Are You trying to learn about Electronics, or, do You just want a flashing LED ?
I'm trying to strictly learn electronics at this point. No particular project in mind. Just playing with well-known circuits.

#### MrChips

Joined Oct 2, 2009
30,467
I'm trying to strictly learn electronics at this point. No particular project in mind. Just playing with well-known circuits.
In my books that is not a well known circuit. I suppose it would work but I have never seen anyone do this before.
It is not exactly electronics.

#### Rich2

Joined Mar 3, 2014
254
I'm trying to strictly learn electronics at this point. No particular project in mind. Just playing with well-known circuits.
Look up the 555 chip. It's very versatile and there's hundreds of circuits available free online.

#### sghioto

Joined Dec 31, 2017
5,369
If you have a NPN transistor try this circuit. Should give appx 1 second ON and OFF.

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#### MrChips

Joined Oct 2, 2009
30,467
Tridon is large after-market automotive parts manufacturer. They used a 555-timer IC for their intermittent windshield wiper controller and flashers.

#### crutschow

Joined Mar 14, 2008
34,044
I'm trying to strictly learn electronics at this point.
Then you first need to understand Ohm's law and how to apply it.
You seem to be a little unclear on that.

#### gerases

Joined Oct 29, 2012
186
Thanks guys, I do know there are more elegant ways to achieve this, but this is where I'm starting after a long break and I'm just having fun. Blinking the light is not the goal really. It's just playing with the breadboard and this is simple enough for me to understand.

#### gerases

Joined Oct 29, 2012
186
If you have a NPN transistor try this circuit. Should give appx 1 second ON and OFF.
That's pretty cool, but I don't quite
If you have a NPN transistor try this circuit. Should give appx 1 second ON and OFF.
Yeah, that's pretty cool but I don't quite understand how this works yet

#### MisterBill2

Joined Jan 23, 2018
17,790
This is a circuit to demonstrate RC time constant, but it seems that the explanation has been rather inadequate, And the relay really has a much lower resistance than would be good for the explanation.
When power is first applied the capacitor starts charging through the 100 ohm resistor, but the current is also flowing through the relay coil. If the voltage ever reaches a value high enough to engage the relay, then the charge will stop and the relay will discharge through the coil resistance until the relay releases, at which time the cycle will start to repeat.

#### MrChips

Joined Oct 2, 2009
30,467
If you wish to learn about RC time constant, charging and discharging, then read up about 555-timer circuits and how they work. There are a lot of sites with good visuals that demonstrate this.