Announcer DoorBell

Posted on Sep 13, 2012

M1 is a clamp connected to the 24V (dc or ac) power from the gate opener. The diode bridge rectifies the alternate voltage (should the opener use dc current, it merely adjust the positive and negative rail to match circuit polarity), in order to get the power required by the 24 V relay. The same potential, limited in current by the series resistor R1, feeds the 5V regulator IC1. The 5V stabilized voltage from IC1 powers the Nutchip; the R3-C5 network ensures proper RESET to the chip. The input stage consists of a double switch relay, RELAY1. The original photocell's output connects to M2, therefore it powers the relay.

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RELAY1 clones the situation it sees on M2: it is energized when the photocell contact is closed, and it is released when the photocell contact is open. As a consequence, relay switches follow the original photocell behaviour. One of the two switches available in RELAY1 is brought to M3, in order to act as a replacement of the original photocell which is now connected to M2. The other is used internally by the circuit in order to detect the photocell signal. This second switch connects to Nutchip input IN1, letting him to know when someone enters the photocell range. The output stage consists of the relay RELAY2 and its driver transistor Q1. We adopted a 24V dc relay in order not to overload the 5V regulator IC. Limiting the load to the Nutchip alone limits the power required from IC1, making an heatsink unnecessary. It is important to limit heat as much as possible in circuits intended for continuous operation, as the less heat produced, the longer the circuit's life. The clamp M4 connects in parallel with the existing doorbell pushbutton (the doorbell rings when RELAY2 is energized). Al the relay is in parallel, the doorbell will also continue to work as an ordinary doorbell. In case of many people crossing the gate, as in the case of a factory entrance during rush hours, a bell ringing continuously can be annoying. Another case is when someone stops right on the door threshold! In order to make circuit operation useful...

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