# RC Circuit and a light bulb

## Main Question or Discussion Point

RC Circuit
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Hi guys, im trying to built an RC circuit in series to light a bulb intermitently. That is on and off in a time interval. Any ideas?

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vk6kro
You could just put the light bulb in series with an electrolytic capacitor across a power supply.

You will see a flash in the lamp when the capacitor charges or is discharged through the lamp.

Don't discharge it while the power supply is still connected.

I'd try 1000 uF or so.

Instead of using wires to connect my circuit can i use any ionic subtances as a conductor. Or will the resistance be to0 high... im using a 3 voltz battery.

We used to make relaxation (RC) oscillators with a capacitor, resistor, and a NE-51 neon lamp. Occasionally you will see flashing highway markers at night that use this circuit. This cannot be done with an incandescent lamp, however. It takes about 60 or 70 volts DC minumum to make it work.

vk6kro
Yes, any conductor will work, but to get a good flash, you would need a fairly low resistance path.
I suppose very salty water would be OK.

There are other ways of making a light flash. What are you trying to do?

"There are other ways of making a light flash. What are you trying to do?"

This is for a presentation im doing to elementary students about electrity...

What other ways do you suggest?

vk6kro
Ok, if it is elementary electricity, you might not want to use any electronics. That would seem like a magic black box.
Electronics would allow you to use tricks like having the flash rate depend on ambient light, though.

You could use a relay.
You get a relay to pull in a set of contacts to remove its own power so it lets go and applies the power again. Just like a buzzer.
If you put a large capacitor across the relay, the action will be slowed down.

You then have another set of contacts on the relay switch the light on and off.
Great for Christmas trees.

just use a very low frequency AC voltage source

vk6kro
The power needed would be pulsing DC.
There should be no voltage, then a suitable DC voltage to run the lamp, then no voltage etc.

There are function generators that can do this, but a simple 555 oscillator would do it for a couple of dollars.
see
http://www.doctronics.co.uk/555.htm [Broken]
You would need an astable circuit for this since it is a repeating effect.

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see
http://www.doctronics.co.uk/555.htm [Broken]
You would need an astable circuit for this since it is a repeating effect.
No. Anything with hysteresis will work. The NE-51 (neon bulb) circuit (see my previous post) works because the NE-51 ignites and conducts when the voltage on the parallel capacitor reaches about 55 volts, then when the capacitor voltage drops to about 45 volts, the lamp extinguishes and stops conducting. Then the capacitor charges again.

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vk6kro