This circuit uses a PIR to detect people entering a room. The PIR usually stays high for as long as someone is present (as long as they keep moving) and is ideal to monitor access to a storeroom or cupboard.

Circuit 1 has a single digit and Circuit 2 shows how to expand the display up to an unlimited number of stage.

Circuit Voltage: 6 to 9 volts

Reset Switch
This can be any push to make switch.

HINT. Connect a 100nF capacitor across the reset switch to create a pulse that resets the display when the power is turned on.

The PIR sensor output goes low when a presence is detected.

The low signal is inverted to a high by the inverter. This is needed because the counter increases on a rising edge, but when the PIR detects someone, it goes low.

The high signal is connected to the clock input of the counter thus increasing the count.

The reset switch when pressed, sends a high signal to the Reset input, resetting the counters to zero.

The second circuit shows how the carry out from one digit connects to the clock input of the next digit. When the digit reaches "9", the Carry goes low. On the next clock pulse the counter rolls over to "0" and the Carry returns high, thus providing a clock pulse for the next digit.

Synchronous and Asynchronous counters.
This version is Asynchronous meaning counters do not increment at exactly the same time, there is a slight delay

• 7 segment displays
• working out what letters can be achieved from a 7-segment display
• Cascading more displays
• Transistor Inverters instead of NOT logic gate.

The PIR can be replaced with other sensors such as an LDR which repsonds to breaks in a light beam caused by:

• movement of objects along a conveyor belt
• movement of people across a path
• a slot in a wheel to count revolutions on a crane spool.

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