# Can I make a gate that will work with AC voltage?

1. May 23, 2015

### Freddythunder

Hello, I remember a few decades ago I built a circuit with my dad using what I think was a 741 op amp and a 555 timer and a loose wire. When you'd touch the wire, an LED would light. Then it took 30 seconds for the circuit to reset. I remember he said it was called a [logic] gate.

Present day, I have some stepkids that ring the doorbell like it's a fricken carnival ride - every morning. Even though I tell them to only ring it once, they are narcissistic and do not care about my piddly request.

I would like to build a logic gate and a timer that only allows the doorbell to ring once every 30 seconds. Here's my caveat. The voltage for the doorbell registers 13.4 VAC when you hold the button down. Can I still use the same kind of circuit (supplied voltage different of course) with a 555 to accomplish my goal?

Thanks,

2. May 23, 2015

### stedwards

I can't think of a way to do this without a dozen parts.

Putting a positive temperature coefficient (PTC) thermistor in series with the door bell switch might be a partial solution. With the right value part, the longer and more frequently the bell is rung the weaker the noise.

The thermistor heats up as current passes through it. This causes the resistance of the thermistor to increase. This reduces the current to the door bell. They are used to protect motors from stall, such as the power window motor in a car.

Last edited: May 23, 2015
3. May 24, 2015

### davenn

yes that is very easy
The output from the op-amp and timer cct would drive a relay that would switch the AC voltage to the bell
do you still have the original cct from your dad ? I or some one else may be able to modify it

if not, could come up with a replacement :)

Dave

4. May 24, 2015

### Freddythunder

Oh yeah, I could use a relay becuase that's just a coil of wire just like the doorbell. Which makes me wonder, maybe I can supply DC power for the circuit and the doorbell and then just use the AC input to a relay to hit pin 2 on the 555. I do not have any idea how to set that up and my dad's instructions are long gone. Any help would be greatly appreciated.

Basically, that relay could send triggers constantly, but I just want the output to be one on-off and then ignore all other triggers for 30 seconds or so. Would that be a one-shot timer? I thought of that as I was typing this...

Thanks!!

5. May 25, 2015

### thankz

I'm thinking this can be done with a some nand gates and hex inverters, a cap for the rc time constant, thanks to the guy who told me I was weakish at circuits because I can't picture the circuit off the top of my head.

as for the gate controlling ac see my post:

6. May 25, 2015

### davenn

Not quite what we are looking for

Hi Freddythunder

sorry for taking a couple of days, I'm just out of hospital and also battling a dose of the flu

here's a circuit that should do what you are after

you can derive the 5V rail from the 13VAC using a bridge rectifier a 1000uF electrolytic cap and a 7805, 5V reg
( or a little higher a 7808 - 8V or a 7810 - 10V would be OK)

breadboard it first and see how you go and report back

cheers
Dave

7. Jun 12, 2015

### tech99

Maybe you could simply power the bell from a large capacitor, which is itself charged via a resistor giving a 30 second delay.