Now we look at a classic circuit, this flip-flop circuit is one always one of the first projects.
Youtube video, suitable whether of not you have the board with you.
See board images.
Tutorial guide for when you have the board with you.
A great deal happens on this classic circuit. If you get lost going through it, don’t panic, just revisit some of the previous boards in this section. We really pull it all together here and if you are a bit shaky on anything we have done before, the cracks might appear here!
Let’s go through the board, we have four ammeters across the top letting us see the current flowing through the resistors R1 - R4. Across the bottom, we have 4 voltmeters that give us the voltages across both sides of the 2 capacitors C1 and C2.
Start with all the switches off and the yellow sliders at the bottom. Now turn on the circuit with the big toggle switch. You will see the red LED above R1 turn on. This means the transistor Q1 is switched on and allowing current through, you can see the flow on the ammeter. This happens because we have a voltage on the Q1 base (b) pin which is connected to the supply voltage by the red line and R3. The V2 meter confirmed we have a voltage of just under one volt.
The other transistor Q2 is not switched on because the switch on the green line is switched off. As a result, the green LED by R4 is off. Now look at the V1 and V4 which show the voltages on the collector (c) pin of the transistors. On V1, where the Q1 is switched on, we have a very low reading because most of the voltage is across the red LED and the R1, but V4 is showing about 3 and half volts because Q2 is switched off.
Now turn on the switch above C2. As you do so you might see the green LED flash if it is not already charged. It will charge to the voltage that is the difference between the V2 and V4 meters – nearly 3 volts with the positive on the right. Now hold the red push-to-break switch by R3 down and then turn on the switch connecting the Q2 base (b) pin – this will give a voltage to Q2 and turn it on. You will see the value on V4 drop to the same value as V1. Now look at the reading of V2, that is showing a reading less than zero! Wow, C2 has pulled the voltage down and that has turned Q1 off! Watch V2 and take your finger off the red button switch. R3 is now how charging C2 the other way so the voltage slowly climbs back up to just under 1 volt! When it gets there, Q1 turns back on.
So, C2 turns off Q1 for a short time when Q2 turns on! By turning on the switch by C1 we can make Q2 turn off for a short while when Q1 turns on! Do that and see it flip-flops!
The speed is controlled by the values of R2 and R3 and the size of C1 and C2. The yellow sliders will change the value of R2 and R3 and so vary the speed of the oscillation.
Note how the current through R2 and R3 increases when you change the sliders, that's why the capacitors are charging more quickly. Search the internet for multivibrator transistor circuits and: