Home > Circuit Blox > Doorway Monitor
UPDATED:.23:04 28 June 2014

This uses Ultrasonics to monitor a doorway or an area up to 2 metres. When an object, human or inanimate, passes the sensors, the counter increases.

Altering VR1 will ensure only one count per detection occurs.

Pressing the PTM switch resets the counter and display to zero.

Use 40KHz transducers for the U/S TX and RX

Circuit Voltage: 6 to 9 volts

Reset Switch:
This can be any push to make switch.

Ultrasonic Astable:
Adjust VR1 with a screwdriver to "tune" the transmitter frequency. Move a hand in front of the sensors while tuning, look for the LED on the detector flashing.
Note Astable is shown upside down.

Set the "Range" to 2, (0.05-2.5s) with VR1 about midway.

IMPORTANT: The transmitter and sensor should be placed in parallel pointing to the area they are monitoring.




The astable is connected to a transducer (TX) which continually transmits a 40KHz sound wave.

When this signal is reflected back from an obstacle, the receiver (RX) picks up the signal. The detector responds to changes in signal level and produces a low output pulse.

This pulse triggers the monostable which produces a timed output pulse. This is needed since the Ultrasonic detector produces lots of pulses quickly, when it detects someone or something depending on the surface of the object. Change the time of the output pulse using VR1. Set it to about the same time it takes for one person or object to pass the sensors so that a double-count does not occur.

The high output from the monostable is connected to the clock input of the counter which increases the count. The counters can count up to 99.

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


Synchronous and Asynchronous counters. The difference, advantages and drawbacks, Race-Hazard when detecting logic states.

This version is Synchronous - all the counters and digits change at exactly the same time.

Detecting objects and imaging using Ultrasonics

The display can easily be expanded by connecting the inputs of the next Dual Counter block directly to the outputs of the previous one.


A Driver and Buzzer can be attached to the output of the Monostable to create an alarm for a shop doorway alerting the owner that a customer has come in.

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