We now build a touch sensor circuit that helps understand the operation of another internal component contained in the 555 timer.
Let’s take a look at the schematic. The section to the right should be very familiar. We have the power and reset pins. An LED is connected to the output and we also add an optional filtering capacitor to pin 5. All the magic happens on the left of this circuit.
We know that the voltages at pins 2 and 6 determine the state of the output. When the voltage at pin 2 below 1/3rd of the supply voltage, it will cause the output to go HIGH. When the voltage at pin 6 is above 2/3rd of the supply voltage, it will cause the output to go LOW. We can trigger the 555 timer by placing our finger on the touch sensor. This will create a path to ground and it will lower the voltage at pin 2, switching ON the output. Remember that we need to increase the voltage at pin 6 in order to switch OFF the output. We can do this by using a capacitor, C2.
The 555 timer output will be LOW as long as the capacitor is charged to a voltage greater than 2/3rd of the supply voltage. We thus need a way to discharge this capacitor so that we can reset the circuit. It is only when we do this, that another touch input can be used to trigger the output. The 555 timer has an internal discharge transistor that is meant to be used for this purpose. Let’s take a look at the internal blocks to understand its operation.
This is the discharge transistor and its collector is connected to pin 7, while its base is connected to the output of the flip-flop. Remember that the flip-flop has two complementary outputs. This means that when the output is LOW, the complementary output will be HIGH and this will switch ON the transistor. When the transistor switches ON, it will pull the discharge pin, that is pin 7 to ground. Similarly, when the output is HIGH, the transistor will be switched OFF and the discharge pin will be disconnected from the ground.
The circuit now makes a lot of sense. When we place our finger on the touch sensor, it creates a path between the trigger pin and ground and this causes the 555 output to go HIGH. At the same time, the discharge transistor is switched OFF and capacitor C2 can begin charging through R2. When the capacitor voltage rises above 2/3rd of the supply voltage, it will cause the internal comparator to switch states and this will switch OFF the output. The discharge transistor will now switch ON and this will create a path for C2 to discharge. The 555 timer can be triggered again once the capacitor voltage falls below 1/3rd of the supply voltage.
We can adjust the values of the RC circuit to change the circuit reaction time.
Here’s what the assembled circuit looks like. In this circuit, the 555 timer is said to have a single, stable state which is the OFF state. When we trigger the 555 timer, it switches to the ON state and then reverts back to the stable, OFF state. This is an example of a monostable 555 circuit.