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Help with STEM project


I'm trying to build a knife where the blade retracts when it is touched by a finger. The idea is a circuit that has a single input (which connects to the blade) and when it's touched it triggers a solenoid that retracts to allow springs to retract the blade. 

I've found a couple of examples like this one:

I built the circuit (about a thousand ways) and I can't get it to work at all. 

Is there something off the shelf at RS that will do what I want? 

Thanks very much for any advise you can offer. 

Kindest regards, 


0 Votes


May 6, 2020 08:01

I had a look at the video(s) and sketched out the circuit which is basically a three stage darling hence providing very high current gain.
The question is why did yours not work assuming all was correctly wired?
When driving the motor connecting the leads via the body, this injects a current from the +9V to the base controlled by your body resistance. This could fail to work for several reasons; your skin resistance is too high so not enough current is injected into the base. However a bigger worry is if you connected the leads together you will put 9V's across 3 forward biased junctions almost certainly destroying one or more transistors. The circuit really needs a resistor on the base circuit to reduce the chance of this occurring.
You could check your transistor base emitter junctions with a diode check or resistance check on a DMM.
In the video when the LED flashes this is due to interference pick up, probably mains "hum" or could be RF, hence te flashing, which becomes bright and continuous when the fingers are placed across both leads.
My suggestion is try to get what you have working before looking for something else as the possibility of mains pick up triggering the solenoid may not be desirable (or it could be what you want!).
What is the solenoid you have selected regarding voltage and current? Depending on this you may find something like allows you to miniaturise and simplify the circuit, it does have a gain of about 20,000 which I assume could be enough. Unfortunately, I could not find any through hole package for this.

0 Votes

May 6, 2020 07:50

@Boss Thank you for your response. I understood about 50%. This is my first project. Sadly I have no idea why it doesn't work and I don't have a diode checker or DMM. I will go through piece by piece with an ammeter and see if I can spot where I'm going wrong. May have blown the transistors as you say. Hopefully this will help: Solenoid - 6v 300mA - JF-0530B Battery - 9v resistor - 1k transistors - C1815 Apologies for my lack of knowledge and thanks again for any help you can offer.

May 20, 2020 14:21

@Dan24 I have been trying to think of ways to help, but it's difficult without actually me doing some hands-on work and unfortunately do not have time! However, I found these links: This shows some touch switch circuits that I would think would be more reliable than the one you are considering and may help in understanding the methods. I also searched for any cheap boards but all have the capacitive sensing on the pcb and only drive a logic level for use by a microcontroller, but I did find this one which will drive a relay, but it still has the sensor on board ... NOTE I have no idea if it will work in your application or if you are allowed to use pre-built circuits! Search eBay for "Capacitive Touch Switch Sensor Module Push Button Key Latch Relay 3A DC 6~20V" Or for general options search "Capacitive Touch Switch Sensor Module"

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May 5, 2020 08:22

HI, Are you looking for a solenoid from RS? You would need to know the voltage of the power supply, the stroke, the force and whether it is a pulling or pushing force as I'd expect you to be using a mechanical linkage to gain a mechanical advantage.

The main issue I can see it the size of a standard solenoid fitting within a knife.

May 5, 2020 08:21

@davecole Hi Dave, Firstly thank you for coming back to me. I've got all the parts but I can't get the circuit from the video to work. I wondered if there's a single component that I can buy that takes care of the touch sensor side of the circuit? The prototype doesn't have to have all of the components within the knife handle. Thanks, Dan

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