Sensor Circuits


Pressure Sensor Alarm

General purpose circuit of the simple pressure sensor alarm is built around a couple of readily available cheap components. Working of this circuit is straight forward and self-explanatory. When the circuit is powered by a 9V compact battery, the active piezo-sounder at the output of IC1 starts beeping for a short time and then goes […]


PC Heat Monitor

The PC processor generates very high temperature during its operation which is dissipated by the large heat sink placed above the processor. If the heat sink assembly is not tight with the processor or the cooling fan is not working, PC enters into the Thermal shutdown mode and will not boot up. If the PC […]


Pseudo Track Occupancy Detector

Track occupancy detectors are needed for hidden yards and other sections of track that are hard to see, but they are also necessary for block operation. The circuit described here uses an LED to indicate track occupancy for digitally controlled Märklin HO model train systems (including Delta Control). In contrast to a real track occupancy […]


Simple BFO Metal Locator

This circuit uses a single coil and nine components to make a particularly sensitive low-cost metal locator. It works on the principle of a beat frequency oscillator (BFO). The circuit incorporates two oscillators, both operating at about 40kHz. The first, IC1a, is a standard CMOS oscillator with its frequency adjustable via VR1. The frequency of […]


Acoustic Sensor

This acoustic sensor was originally developed for an industrial application (monitoring a siren), but will also find many domestic applications. Note that the sensor is designed with safety of operation as the top priority: this means that if it fails then in the worst-case scenario it will not itself generate a false indication that a […]


Inductive Speed Sensor For Cars

Here’s a way to avoid winding the pickup coil for the inductive speed sensor used in projects such as the Speed Alarm, Gearshift Indicator, etc. All you need to do is carefully break open the plastic case of a small relay and cut away the armature mechanism, so that you are left with the relay […]


Two-Wire Temperature Sensor

Remote temperature measurements have to be linked by some sort of cable to the relevant test instrument. Normally, this is a three-core cable: one core for the signal and the other two for the supply lines. If the link is required to be a two-core cable, one of the supply lines and the signal line […]


Infra-Red Sensor/Monitor

The sensor/monitor shown in the diagram ‘wakes up’ the host system on detection of infra-red (IR) signals. It draws so little supply current that it can remain on continuously in a notebook computer or PDA device. Its ultra-low current drain (4µA maximum, 2.5µA typical) is primarily that of the comparator/reference device, IC1. The circuit is […]


Tiny Dew Sensor

Dew (condensed moisture) adversely affects the normal performance of sensitive electronic devices. A low-cost circuit described here can be used to switch off any gadget automatically in case of excessive humidity. At the heart of the circuit is an inexpensive (resistor type) dew sensor element. Although dew sensor elements are widely used in video cassette […]


LM35 Two-Wire Temperature Sensor

The Type LM35 temperature sensor from National Semiconductor is very popular for two reasons: it produces an output voltage that is directly proportional to the measured temperature in degrees Celsius, and it enables temperatures below zero to be measured. A drawback of the device is, however, that in its standard application circuit it needs to […]