Temperature Relay Circuit


This is a design circuit for temperature relay that can be used to signal a fire or set point for temperature monitoring function. You need to adjust P1 so that T1′s base voltage is 0.5V smaller than the emitter voltage at a temperature a little bit lower than the desired triggering (switching) temperature. This is the figure of the schematic circuit;

If the temperature increases then T1 and T2 start conducting and the relay is closed (ON). If you want to use it as a cold relay or to signal an inferior temperature limit, then Th1 and P1 change places. After the relay is triggering you need to open switch S1 in order to stop the circuit. The nominal value of P1 must be chosen according to the used NTC thermistor and the switching temperature to be adjusted.

Tilt Sensor Alarm Circuit


This is a design for simple circuit of the tilt sensor alarm presented here can be fabricated using readily available inexpensive components. The circuit is a true transistor based design. Home made Tilt sensor for this circuit is an ordinary little glass/plastic bottle with two metal needles inserted through its cap, and a small quantity of water inside. A 9V alkaline battery is enough for powering the whole circuit. Here’s the figure of the schematic circuit;

In this figure shows that this circuit is based on two transistor as main control. There are BC 557 and BC 547. Usually, transistor T1 is in inactive state. When the sensor assembly is tilted, both needles inside the sensor (bottle) are short circuited by the water and a positive voltage is available at the base of T1 and it becomes active. Activation of T1 causes the activation of next transistors T2 and T3. After this, T2 supplies constant bias for T1 to make it latched and T3 triggers the SCR(T4) which in turn energizes the active piezo-sounder(BZ1). Once activated the circuit can be deactivated by depressing the power/reset switch S1. Preset pot P1 is deliberately added here to adjust the circuit sensitivity. This may become necessary if you are trying a different (readymade) tilt sensor. Similar, SCR(T4) and piezo sounder (BZ1) may be replaced with near equivalent parts. Resistor R3 (100-150 Ohm) is optional.

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