Noobie question on how this circuit works

Hey Everyone!

I had some time today and was toying around with a electric brush that was lying around. The brush has a single push button switch which has 3 states - ON(low speed)- ON (high speed) - OFF. There is a diode attached to the circuit which I’m guessing is reducing the voltage for the low speed and then is excluded at higher speed. I want to know how the push button works this way. I read up about SPDT switches and looks like it is use in this circuit. But the wiring is kind of confusing. Can someone please explain to me how the circuit is completed in the 3 states ?

I have attached photos of the actual product and the board. And a poorly drawn circuit diagram showing how wiring is done.

Thanks!

adding another photo

Hi,
I would say you have got the circuit description correct, the diode just provides a volt drop to give low speed.

The 3 click button will have some mechanical arrangement that provides the ON-ON-OFF action from the button.

Haven't seen any description of how one works, but they are in all sorts of LED lights and toys these days.

Tom... :slight_smile:

thanks Tom,,

can someone who has worked on this type of circuit explain how the actual circuit is completed in the low power and high power settings?

thanks

Hi,
It looks like the motor is powered by 2 x 1.5v batteries.
So the motor is probably rated at 3V full power.

The switch in high speed connects the motor directly to the batteries, so it gets the full 3V

In low speed the diode is place in series between the motor and the battery in what is called "Forward Bias" mode.
The white bar end of the diode is the Cathode end and it will be connected so it is pointed to the negative part of the circuit.
The other end is the Anode and it is pointed towards the positive part of the circuit.

With the motor between the battery and the diode, the motor now receives 3V - 0.7V = 2.3V, so it runs slower.
The diode when conducting current in "Forward Bias" mode drops 0.7V approx across itself, so the remaining 2.3V is across the motor.
The 0.7V is a good portion of the motor supply so you see a significant drop in speed/power.

If the system was 12V, the diode would not affect the speed much because it would still only drop 0.7V which would leave 11.3V for the motor, not much change from 12V.

Tom.. :slight_smile:

Thanks Tom… that is exactly what is happening… my question is how does the spdt switch achieve this ? Can you explain which contacts open and close on which operation using the circuit shown in this attachment ? I’m having a hard time wrapping my head around it.

Thanks,

Raam

Quite possibly a very simple IC that's straight on the board under a gob of epoxy that you'll never get to. Lots of cheap products do it that way.

not sure where you’re suggesting it is located… there are only 2 components on the board 1.spdt switch 2.diode

Hi,
2 (2)edit.jpg

Tom… :slight_smile:

Ok the magic is in the button, it's one of these but you can keep digging around for the internals

You have a better idea of what it could be seeing as how you know how much it travels, what it feels like, etc. I imagine it could just be like a ballpoint retractable pen with a staggered rotating crown.

INTP:
You have a better idea of what it could be seeing as how you know how much it travels, what it feels like, etc. I imagine it could just be like a ballpoint retractable pen with a staggered rotating crown.

Good thinking... :slight_smile:

Yeah, the assumption of it being 'momentary' is misleading. Just an illusion. It is internally actually a latching behavior switch. Only difference is the button shell pops back up just a pen's button does on some pens (like my EDC pen of choice, a Pilot G2 Limited)

Okay, so I got a chance to open the switch and it is similar to what INTP suggested. It is a latching mechanism which is similar to the ones in ball point pen. The metal contact switches between the two pins on either ends for the first 2 clicks and loses contact to both during 3rd attempt creating ON/ON/OFF positions. The low power mode uses the diode to drop the voltage and high power mode skips it and is powers the motor directly.

Thanks for the directions INTP and Tom. You guys rock!

Learnt something today!