[www.keralites.net] Freezer Monitor Alarm

 

Freezer Monitor Alarm

All items stored in a deep freezer will thaw out if, for some reason, the temperature inside the freezer rises to the thaw point. However, a freezer monitor alarm can warn you of the rising temperature before the thaw point is reached.

This simple circuit is powered by regulated 9V and built around a few discrete components that are readily available in the market. Current consumption of the circuit is only a few milliamperes in idle state. So you can also use a 9V 6F22/PP3 type compact battery for powering the circuit.

Fig. 1: Freezer alarm circuit

Germanium transistor AC125 (T1) acts as the temperature sensor. For sensing, the monitor circuit uses the leakage current of the transistor, which rises with temperature, and is adjustable from –18°C to room temperature.

Working of the circuit is simple. At normal temperature of the freezer, the resistance between the emitter and collector of transistor AC125 is very high and therefore the voltage at the gate terminal of SCR BT169 is too low to fire it. As a result, the rest of the circuit is disabled. Relay RL1 does not energise and hence the buzzer remains silent.

Fig. 2: Pin Configurations of Bt169 and Bs170

When the temperature of the freezer rises beyond the level fixed by preset VR1, the gate voltage of SCR BT169 (T2) goes high and the SCR is fired. Consequently, the gate terminal of MOSFET BS170 (T3) receives positive voltage through resistor R2 and relay RL1 energises to latch. The buzzer sounds to indicate higher temperature of the freezer, unless you switch off the circuit through switch S1. The relay contacts can also be used to control an emergency sounder, signaling beacon, etc.

Assemble the circuit on a general-purpose PCB and enclose in a small cabinet such that it's moisture-proof. Take out two wires from the cabinet for connecting the sensor. Insulate sensor leads to avoid shorting and place inside the freezer to monitor the temperature. You can replace the transistor sensor with an AA115 diode or even a small NTC type thermistor.

---

www.keralites.net

__._,_.___

Reply to sender | Reply to group | Reply via web post | Start a New Topic

Messages in this topic (1)

Recent Activity:

• New Members 37

Visit Your Group

KERALITES - A moderated eGroup exclusively for Keralites...
To subscribe send a mail to Keralites-subscribe@yahoogroups.com.
Send your posts to Keralites@yahoogroups.com.
Send your suggestions to Keralites-owner@yahoogroups.com.

To unsubscribe send a mail to Keralites-unsubscribe@yahoogroups.com.

Homepage: www.keralites.net

Switch to: Text-Only, Daily Digest • Unsubscribe • Terms of Use

.

__,_._,___