Electric bell for shops

Bells in shops are coming back into fashion, which will ring when the door opens, so that the staff is informed of the arrival of the customer (or even wake them up from hibernation…).

Connecting the doorbell

I decided to replace the classic doorbell with an electric one. I powered my little bell from a 6V adapter. The current passes through the capacitor, which charges for a certain time and therefore only a short ringing is heard from the doorbell. There is a discharge resistor in parallel with this capacitor, which has the task of discharging it immediately, to be ready to ring the bell again with an incoming new customer.
A switch is also connected in parallel through this capacitor and resistor, which is turned on only at night and in the event of unauthorized entry into the store and opening the door, the bell will start ringing without a break (unless the offender breaks it….).

The component values ​​during testing were set at 500uF and 1k5 resistance. Well, the values ​​will probably be different for each bell.

But what if the store staff wants the door open in the summer? It will be necessary to replace the optical switching. Laser diode (e.g.. from the pointer) illuminates the phototransistor (or even a photoresistor), which turns off the switching transistor. After the customer passes through the door, the laser beam is interrupted for a short moment, the switching transistor turns on and a short ringing is heard from the doorbell again.

For long laser life. diodes, I recommend reducing the current flowing through this diode to a minimum, at which the phototransistor is able to work. The laser diode should be powered by pulse voltage – not unidirectional – it reduces its lifespan. It may also be sufficient to use a pulsating voltage from the adapter (50Hz/100Hz) unfiltered by electrolytic capacitors. Thyristor v zapojení je pre nočnú prevádzku (against robbery) and the diode connected finally to the ground protects the transistor and thyristor against spikes arising when the bell rings.

The diode in the control electrode of the thyristor is therefore, so that Ty is not switched off by the resistor, which is connected to earth. Who would like to save one part, it is possible to connect a trimmer instead of a discharge resistor (for me the 1d5) and from its runner lead the control current through the diode for the thyristor. This eliminates the need to fit a resistor for the control electrode and the current is set by the trimmer runner. As a transistor, it will be necessary to use some darlington or ordinary one with a large h21e. The thyristor can be ordinary, e.g.. KT501 up to 505.

Therefore, the capacitor with a parallel resistor remained in the connection, not to happen, that the customer would remain standing in the place of the laser beam and the bell would then ring to taste.

I have not tried this connection, it's just my proposed solution. I will answer any questions at rado_pro@zem.sk. (I do not answer burglars – except for Jánošík…)

Thank you Rado for the interesting idea!