It’s been a while since our last basic components tutorial, right? Last time we have seen the operational amplifier and how to use it. Well now, this is a Schmitt trigger inverter, for the tests the CD40106B and that’s what we will see today. So, let’s get started.
Let's say we have a AC signal taht we want to encode, so count the frequency or the pulses. But the wave is not perfect because it oscillates with some noise. Using the OPAMP with a set threshold we would get a high pulse each time the AC signal pases that threshold. But, if we have noise, that could trigger short pulses as well and we don't want that. That's why we need two threshold values.
The Schmitt trigger has just that. Two threshold values, lets say 3V and 2.8V. The input gets over 3V and then the output is High, then the input has a ripple and gets a bit below 3V, but since it didn’t pass below the second threshold, the output didn’t change neither. So now we get the good output that we want. So, that’s the main function of a Schmitt trigger, to convert a noisy signal to a good square wave that could be then read by a microcontroller or other digital components.
Ok, so let’s say you don’t have a Schmitt trigger IC, you only have OPMPs. Well, here is a Schmitt trigger configuration made with an OPAMP. Negative input connected to ground so that implies that the V+ point will be ground as well.
So, we would have these equations above. The current that passes R1 is the same as in R2 and since V+ is zero, we get that Vin divided by R1 is equal to negative Vout divided by R2. So, Vin is equal to negative R1 divided by R2 and multiplied by Vout where Vout could have values of the minimum and maximum supply voltage. In this case let’s say the OPAMP is supplied with plus minus 5V, so Vout could be plus minus 5V. Also, let’s say that R1 is 1k ohms and R2 is 1.7K ohms. So, we get that for the switch to occur, we need Vin to be equal to 1k divided by 1.7K and multiplied by plus or minus 5V, so the switch will be at plus minus 3 volts, right? So, there we have our two threshold values. When the input is rising from negative 5 volts to positive 5 volts, when we reach positive 3V, the output will be high. But on the falling edge, when we get from positive 5v to negative, only when we reach -3V, the output will be low. And here is why the icon of the Schmitt trigger are these two threshold values.
When the input is rising from negative 5 volts to positive 5 volts, when we reach positive 3V, the output will be high. But on the falling edge, when we get from positive 5v to negative, only when we reach -3V, the output will be low. And here is why the icon of the Schmitt trigger are these two threshold values.
But what if we want different threshold values, let’s say positive 3.5V and positive 2 volts. For that we use this configuration with 3 resistors. Here we have the equations for rising and falling threshold values and let’s say that we supply our OPAMP with 5V.
When Vout is ground, we actually have both R3 and R2 connected to ground, so it is the parallel of those resistors. So, we get a voltage divider like the one in the equations above. So, let’s say the resistors values are R1 equal to 6.8K, and R2 and R3 equal to 10k. We get the falling edge threshold of 2.1V. And when Vout high, so 5V, we get the other equation. So, with the same resistor values, we get a threshold voltage of 3.5 volts for the rising edge. So, there you have it, the rising value is 3.5 volts and the falling threshold value is 2.1 volts, and we’ve got our Schmitt trigger made with OPAMPs. Fine tuning the resistors values, you could set the threshold.