# Electronic – Using capacitors for flashing lights

capacitorledtransistors

I am a beginner!

I've been trying to devise a very simple circuit to confirm in my mind how capacitors work. The goal is to make an LED flash on and off using a capacitor or two and one or more transistors.

I've tried various things but I'm getting nowhere fast. I've seen circuits online but they tend to involve two LEDs, inductors or other strange setups.

Can anyone explain the theory behind what I'd need to do (without delving into the maths too much please)?

As far as I can tell a capacitor could be used to control a transistor to make it switch off after a second or so.. my problem is getting it to turn back on!

If anyone can describe what needs to happen (logically) in order to make the LED turn on and off that'd be really good!

Cheers,
John.

What you've seen with two LEDs it is called an astable multivibrator.

It has two states that change whenever one of the capacitors is discharged.

Assume Q1 is ON. This means the positive side of C1 gets connected to ground. This leads to reverse charging of it (discharging), thus modifying the voltage at the negative side - it becomes positive and biases Q2 which turns ON. Q2 starts discharging C2 and it turns ON Q1 again. And this cycle repeats forever.

While either of the capacitors is charged, the base voltage of the transistor is negative, thus the transistor remains OFF. The frequency of this is related to the time needed for the capacitor to discharge (when it is connected in reverse polarity to ground via a resistor - e.g. C1-R2).

Source: Wikipedia - Astable Multivibrator

LED Flasher circuits usually work on the same principle.

Source: next.gr

When they have one LED, the other is usually replaced by a resistor.