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Electronic Schematics - F

Freezer Alarm Circuits

Cold Activated Switch - A simple thermistor triggered switch with adjustable threshold. It triggers with cold temperatures so may be used as a frost alarm or cold temperature switch.

Freezer Door Alarm:

Frost Door Alarm - The freezer door alarm has been working great, except for one minor issue. When loading the freezer it was often open for more than the 27 seconds the timer was set for. So while loading the freezer (or staring blankly into it deciding what you wanted) you’d either have to close the freezer door to reset the timer, or suffer through the buzzing.

Sensitive Buzzer Freezer Alarm - This is a freezer alarm circuit, but ultra sensitive using a buzzer. The circuit raises an audio notify when the temperature inside the freezer box goes up above a preset level. The circuit needs 9V dc provide and may energy from the normal 9V alkaline battery. A negative temperature co-efficient form (NTC sort) little thermistor (R1) here works because the freezer standing sensor. Associated elements (T1 and T2) are utilized to drive a 6 to 9V rated mechanical buzzer. Once the sensor detects a temperature shoot, the buzzer begins beeping and stays in that problem, until energy towards the circuit is removed. For this on/off additionally reset purpose any little dc toggle swap is usually inserted at the optimistic rail.

Fridge Door Alarm Circuit - This fridge door alarm is using a 3V battery supply should be placed (in a small box) in the fridge near the lamp or close to the opening. With the door closed the photo resistor R2 presents a high resistance (>200K) thus clamping IC1 by holding C1 fully charged across R1 and D1. When a beam of light enters from the opening, or the fridge lamp lights, the photo resistor lowers its resistance (<2K) stopping C1 charging current. Therefore IC1, wired as an astable multivibrator, starts oscillating at a very low frequency and after a period of about 24 sec. its output pin (#3) goes high, enabling IC2.


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