You Have Mail!

Posted on Feb 5, 2014

If your letterbox is some distance from your house, you will find a monitoring device useful to indicate when new post has arrived. This can take the form of a US-style visible flag; a more modern alternative uses a 433 MHz radio transceiver. The big advantage of the solution presented here is that is can use an existing two-core bell cable, witho

You Have Mail!
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ut requiring any further power source. The arrival of post is signalled by a blinking LED; for added effect, a digital voice recorder can also be connected which will, at regular intervals, announce the fact that the letterbox needs emptying. The device is silenced by a reset button. The circuit uses one half-cycle of the AC supply to power the bell or buzzer, and the other half-cycle for the post indicator. Suitably-oriented diodes in the device and in the letterbox ensure that the two signals are independent of one another (Figure 1). The bell current flows from K1. A through D3, bell-push S2, D1 and the relay back to K1. B. C1 provides adequate smoothing of the current pulses to ensure that the relay armature does not vibrate. The bell is operated by the normally-open relay contact. If the bell is actually a low-current piezo buzzer, then it can be connected directly and the relay dispensed with. During the half-cycle for the letterbox monitor current flows from connection K1. B on the bell transformer through current-limiting resistor R1, the LED in the optocoupler, reed contact S1 (a microswitch can also be used) and D2 and finally back to K1. If the reed contacts are closed, the LED in the optocoupler will light and switch on the phototransistor. A positive voltage will then appear across R3 which will turn the thyristor on via C6. The red LED will indicate that post has arrived. Pressing S3 shorts out the thyristor,...

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