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Devices Electrotechnica Elektrokit
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Mail Indicator With Warning

Price : INR 470.00

Mail Indicator With Warning is specially designed for people, who make several futile trips to their mailboxes before their mail finally arrives. The idea is typically based on a sensor placed in the mailbox and the sensing circuit using IC40106 (IC1).


Mail Indicator With Warning consists of an LDR i.e. Light Dependent Resistor. When the letter is dropped in the mailbox, light falling on LDR is interrupted; its resistance rises and triggers the sensing circuit. The circuit will generate a noticeable alarm tone and simultaneously begins to flash two visual indicators.

About the Circuit :
The heart of the Mail Indicator With Warning is IC1, which consists of six Schmitt trigger inverter gates. The sensor assembly consists of an LDR and a light source. The light falling on LDR keeps the output at pin2 is at logic low level. Diodes D1, D2 remains conducting and so the red ‘NO’ mail indicator LED D5 remains lit, and piezoalarm tone oscillator remains deactivated.

However, the moment a letter or a packet is dropped in the mailbox, the red ‘NO’ mail steady indication ceases. In turn oscillator designed around one of the inverter using R2/C1 operating at a visually attractive frequency of about 2 Hz and the set of green LEDs D3 and D4 indicating the arrival of ‘MAIL’, begin to flash. Simultaneously as D2 ceases to conduct, two inverters whose inputs are pin5 and 9 and outputs are pin6 and 8 simultaneously, both of which are wired in parallel to provide current drive boost, begin to oscillate at about 2KHz.This frequency is set by capacitor C2 and resistor R5.

Calibration :
The sensitivity of the circuit is set by VR1. Adjust VR1 such that the alarm tone just goes off in the normal ambient light conditions. Next try putting a letter/envelope and ensure that it does set off the alarm.


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