We had already seen the astable mode operation of 555 in my previous blog. Here, we will design a circuit for bistable operation of 555. Bistable operation of 555 is more simpler than astable operation. In bistable operation, output pin ( pin 3 ) will become HIGH, if the TRIG pin ( pin 2 ) is at LOW voltage. Similarly, output pin ( pin 3 ) will become LOW, if the RESET pin ( pin 4 ) is at LOW voltage. If both pins are at LOW voltage, higher priority will be for RESET pin ( pin 4 ) and output pin ( pin 3 ) will remain LOW.
Pull down resistors ( R1 and R2 ) and push button switches ( S1 and S2 ) are used to control the input voltage levels of TRIG pin ( pin 2 ) and RESET pin ( pin 4 ). R1 and R2 are the pull down resistors connected to RESET pin ( pin 4 ) and TRIG pin ( pin 2 ) respectively. S1 and S2 controlls the voltage levels at RESET and TRIG pins.
Output pin ( pin 3 ) is connected to an LED through a current limiting resistor. DIS pin ( pin 7 ) is left disconnected. THR pin ( pin 6 ) is grounded. A small capacitor is connected across CTRL pin ( pin 5 ) and ground. 5V is normally supplied from a 5V regulator. Pin out diagram of 555 is given below.
S1 and S2 are OFF by default. When S2 is pressed, S2 turns on and TRIG get grounded. When TRIG get a low voltage, output will shift from LOW voltage to HIGH voltage. This will turn on the LED. When press is released, S2 will turn off . But the output will remain HIGH and LED will remain in the ON state. Now, press S1. This will turn on S1 and RESET pin get grounded. A LOW at RESET pin will result in LOW voltage at OUTPUT. This will turn off the LED. LED will remain in the OFF position, even if the press is released.
Pull up resistor is very important in this circuit. If pull up resistor is not connected, when any of the switch is pressed, power supply get shorted through the switch. High current will flow through the circuit due to low resistance of the path. This may damage the power supply.
0 comments: