This circuit might be nearly 200 years old, but knowing what its does and how it does it expands our understanding of potential dividers.
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.
The idea behind this circuit is nearly two hundred years old and even more impressive is that it’s still used in sensor circuits today! It’s called a Wheatstone Bridge. So, what’s it all about? Let’s look at the board. The left and right-hand sides have the same layout – it’s two potential dividers (two resistors in series). It looks more complicated because at the top and the bottom we can select the resistor we use – this gives us different values to play with. Note the names of the resistors, R1 and R2 on the left and R3 and R4 on the right. In the middle of the board, we have a centre point voltmeter showing us the voltage difference between the middle points of the two potential dividers. Why we would want to do this will become apparent and is really the purpose of the circuit.
Start with all the small switches positioned on their left side and power up the circuit with the big toggle switch in the top left corner (you can hold down the green button if you wish instead). Three digital voltmeters will light up. The one at the top is giving us the supply voltage, the red one on the left shows the voltage at the point where the two resistors R1 and R2 connect. The green meter on the right does the same for the point connecting R3 and R4.
Look at the R1 and R2 resistors, they are both 200 ohms. When we have a potential divider with resistors of the same value, the voltage is split evenly over each one. We can check this with the voltages shown on the white and red meters. The red one will be half the white one. The values might not be exact because the resistors and meters can be slightly inaccurate but they will be very close.
Now look at the right side of the board, here the R3 resistor (75 ohms) is half the value of R4 (150 ohms), so the voltage across R3 will be half the voltage across R4. In other words, one third of the total voltage will be across R3 and two thirds across R4 because the voltage ratio is always the same as the resistance ratio. Used the white and green meters to check this.
Next look at the panel voltmeter in the middle, it’s unusual to have a meter not wired up with one connection to the negative supply (ground), but that is the case here where it connects to the middle of each potential divider. This means that voltage displayed is the difference between these points. You can check this be working out the difference between the red and green meters and compare it with the panel meter voltage. It should be just under 1 volt.
Now toggle the top right switch (S3) so the 150 ohm resistor is used. Now both R3 and R4 resistors are the same value, so the voltage across each will be half the supply. As the voltage is the same as the other side of the board now, the panel meter is reading zero. The resistor values on this right side of the board (R3 and R4) are different to the two on the other side (R1 and R2), but that only effects the current. The voltage is the same because the resistance ratios on both sides of the board are the same, a ratio of 1:1. Change the switches so that R1 is 100 ohms, R2 is 33.3 ohms, R3 is 75 ohms and R4 is 25 Ohms. Now the ratio on both sides is 3:1 and you can check this with the digital voltmeter values, but importantly the panel meter is reading zero again.
So - and this is the important bit - if the panel meter is reading zero, the voltage in the middle of each potential divider is the same and this can only happen when the resistance ratios as same. This is really important for understanding the next board.
Find all the different ratios you can where we have the same on both sides. Remember the ratios will always be the same if the panel meter reads zero volts
At first glance this board might look very different to the last one, but we still have a Wheatstone Bridge circuit here. The selector switch means we have 5 options for R1. The idea here is that we can work out the value and then check it by moving the wooden slide to uncover the resistor. Below R1, we have R2 and this can be either 10 or 100 ohms. Now R3 and R4 are shown as a variable resistor. This is the very thin (0.1mm) nichrome wire at the bottom of the board. It is so thin it has a resistance of about 1 ohm per centimetre. You need to use a banana lead to connect the green socket and the wire. By sliding the plug along the wire, we create the variable resistor and can change the values of R3 and R4. If this doesn’t make sense, don’t worry (at the moment).
Move the selector switch to position A, ensure R2 is switched to 10 ohms and power up the board with the big toggle switch. Now slide the plug on the end of the banana lead along the thin wire, you will see we get a voltage in the digital meter and this changes as we slide. Move the plug along the wire until the digital meter reads zero volts. This might be the top or the bottom wire, you will see they are connected on the lefthand end. At the zero volts point, moving it one way will show a negative voltage and the other way will show a positive one.
We learnt from the last board that if the central meter is zero then in means that the resistance ratios on both sides of the board must be the same. Now the top right side of this variable resistor is the supply voltage and the bottom right side is ground. With the left side of the two wires being connected, it means the whole length is 40 centimetres (cm). The position of the banana plug on the wire is about 13 cms from ground (point A) and 27 cms from the supply (point B). So R3 is 27 cms of the resistance wire and R4 is about half that at 13 cms. At 1 ohm per cm R3 will be 27 ohms and R4 about half that at 13 ohms. That gives us a R3/R4 ratio of about 2:1. We know the ratio on the left side must be the same as the meter is zero, so if R2 is 10 ohms then R1 must be 20 ohms. Move the wood to check this.
Have a go at working out the resistance of the other 4 mystery resistors in the same way. If you find the zero point is very close to the ends of the wire on the righthand side, try changing the value of R2 to the other option - it might help. Note that I have written in the number you need to multiple R2 to get R1 on the board. For example, by the 13cm point it has x2 indicating that R1 was twice R2. Something the value will not be exact but as long as you are close that’s OK.
Research the history of the Wheatstone Bridge. You will see what I mean about the first version of it being nearly 200 years old.
Do an online search for Wheatstone Bridge temperature sensor circuits and you will find examples of how this circuit is still used today.