# How to turn on and off with certain frequency (for example 20 Hz) a simple circuit?

I'm trying to connect a battery of 9 V to the secondary coil in a transformer (220 V - 12 V), but I would like to turn on and of this connection with certain frequency in order to see in a oscilloscope what happens in the secondary coil (sorry if my English is bad). I attached an image to explain myself better.

I would really appreciate if someone could help me to do this.

As Vladimir said, I want to power the transformer through the battery and when I disconnect the battery there would be some current induced in the primary coil. To see this signal in the oscilloscope I think the switching of the battery should be periodic (correct me if I'm wrong here), but I don't know what should the frequency be exactly. This is the reason why I want to turn on and off the battery with a certain frequency. Finally, I made another sketch that I hope is more readable.

If you could guide me and tell me if I'm doing the right thing to see the desired signal, I would really appreciate it.

• We used to use something called a "vibrator". You can see one at this YouTube video where I've positioned the video right at the point where he is opening one up. That mechanical technique was used with power transformers "back in the day." So it should work. If you can find one. – jonk Apr 4 at 7:28

To switch the supply at 0.2 Hz you could use a relay driven by a 555 timer.

Alternatively you could try a simple relay oscillator.

simulate this circuit – Schematic created using CircuitLab

Figure 1. A very crude relay oscillator.

How it works:

• On power-up RLY1 coil is energised causing the relay to start to pick up.
• When the relay armature is partially closed contact 1a will open and cut the feed to the coil. The relay armature will start to release.
• When the armature has released far enough contact 1a will close again and the cycle starts again.
• You would use contact 1b to switch your circuit.
• D1 protects the contact 1a from arcing and also slows down the release of the relay.

Frequency of oscillation will depend on the relay chosen and may not be very repeatable from cycle to cycle.

• Hey, it seems to me that OP wants to power the transformer through the battery, the primary would be open, with the scope probe connected. – Vladimir Cravero Apr 4 at 8:24
• I think you're right. Circuit reads from right to left. I read it left to right and couldn't read the writing due to the low resolution. – Transistor Apr 4 at 8:26
• Even when powering the transformrr from the battery, there has to be a minimum frequency. If the switch would stop toggling, the battery becomes short-circuited – tobalt Apr 4 at 8:29
• @tobalt, at 0.2 Hz that's going to be a problem anyway. – Transistor Apr 4 at 8:31
• The battery is on the less inductive low voltage side of the transformer. Therefore, the minimum frequency is on the order of kHz (50Hz * (220/12)^2 is already 16kHz) – tobalt Apr 4 at 9:52