I was approached by my local maker space, Moore Makers, and asked if I wanted to do a talk or teach a class. I enthusiastically said yes! I offered to put together some kits to build a simple single transistor touch switch. The entire circuit has only three components and can be assembled on a small breadboard. The cost for each kit is $1 to $2 depending on where you source the parts from.
A transistor is a semi-conductor which means that it can vary a current flow from none to a lot. It is really just a switch. It has three leads; a collector, a base and an emitter. When a current is applied to the base the collector and emitter will conduct electricity. This is of course a VERY simple explanation of how a transistor works but it will suffice for the understanding of this circuit.
When your finger touches the two strips of copper wire a small current flows through the skin from the battery positive lead to the transistor base lead. This current can be varied in it’s resistance by how hard you press your finger on the leads and by how moist your skin is. DO NOT allow the full 3 volts of the battery to touch the base! This will destroy the transistor!
To make the kits for this project I cut up two breadboards into smaller pieces with a band saw. Each piece has 25 holes in a 5 by 5 pattern. This is all that is needed to assemble the project on. I cut pieces of foam board into 2″ x 2″ squares then hot glued the mini-breadboards to the foam board. I printed up the breadboard layout sheet on a laser printer 4 up on an 8.5″ x 11″ page then cut them into 4 sheets. The copper wire is a single piece of 20 gauge bare wire cut to about 3″. I’m allowing the builders to cut and bend the pieces themselves but you could also cut and bend them ahead of time. The kit calls for four pieces of wire cut to 1/2″ lengths and bent into a U shape. Two of them are for the contacts and two are to help hold the small wire of the battery leads in place. I ordered the parts from Tayda Electronics for about $30. The batteries and copper wire came from Radio Shack.
Refer to the breadboard layout diagram below for placement of wires and components.
Cut the length of bare copper wire into four 1/2 inch long pieces. Using a pair of needle nose pliers bend two pieces into a U shape the same width as the hole spacing on the breadboard. Bend the other two pieces at a right angle in the middle. Remove the batteries from the battery holder. Insert the black negative battery lead into the breadboard and in the same hole insert one of the right angle pieces of copper wire to help hold the battery wire in place. Insert the red positive battery lead into the breadboard and in the same hole insert the other piece of right angle copper wire. Install the transistor paying attention to the pin placement as shown on the breadboard layout diagram. Bend the leads of the resistor and insert them into the proper holes in the breadboard. Install the LED in the breadboard paying attention to the positive and negative pin placement as shown on the breadboard layout diagram. Insert the U shaped copper wires into the breadboard to serve as the contacts for your fingers. Check to make sure all the components and leads are in the proper place. Check all the bare leads and make sure none of them are touching each other. Once you are sure everything is OK, install the batteries in the battery holder taking care to put them in the right way!
Making it work
Place your finger across the two bare copper wires. The LED should light up. If it doesn’t, moisten your fingertip and try again. You’ll find that the LED will glow brighter with more pressure and dim with less pressure. This is because you are changing the resistance of the current flow and the transistor is “semi-conducting. Experiment by touching one finger on each lead. Now the current is flowing through your entire body. Try touching one lead and have another person touch the other lead. Now touch each other skin to skin and watch the LED light up.
All the parts for this project were ordered from Tayda Electronics. Below is a list of the parts with the Tayda part numbers.
From Tayda Electronics:
1 – BC547 Transistor NPN 45V 0.1A Part# A-137 Price: $0.04
1 – 220 OHM 1/4W 5% Carbon Film Resistor Part# A-2119 Price: $0.01
1 – LED 5mm Red Super Bright Part# A-1554 Price: $0.03
1 – 2 x AA Battery Holder Part# A-746 Price: $0.16
1 – 830 Point Breadboard Part# A-2372 Price: $4.59
From Radio Shack:
Insulated hook up wire (must be stripped) Part# 278-1222 Price: $8.99 for 75 feet.
This week’s project video. Keep on hackin!