Visual doorbell circuit for deaf people

The collector potential of VT2 is equal, the sampling transistors VT3 and VT4 are cut off, and the electronic switch
is closed, so the alarm does not work. When the ambient temperature changes, the bridge is unbalanced and VT. ,
VT2 collector has a potential difference, so VT3 (temperature drops) or VT4 (temperature
rises) is turned on, the corresponding electronic switch is turned on, and an audible and visual alarm is issued.
    Circuit working principle: As can be seen from the figure, T. It is the original 8 V/l A doorbell power transformer
, S. and BZ. They are the original doorbell button and doorbell respectively. This part of the circuit is exactly the same as the
original doorbell circuit . The 8v AC voltage output by L secondary is
rectified and filtered by VD, C: to power the lighting control circuit. When the doorbell button is pressed s. At that time, the doorbell rang
. At the same time, the transistor VT. It turns on and off with the 50 Hz mains frequency
(positive half cycle on, negative half cycle off), and the 50 Hz pulse falling edge of its collector
output is triggered by Ic,. , monostable multivibrator composed of IC ib. The monostable time of the latter
is set to 15 s by C3 and hurricane, and the monostable pulse it outputs starts Ic through the wind. ,
an oscillator composed of ICId. The oscillation signal output by ICid is then used to control the on and off of the drive tube VTz.
As a result, the light-emitting diode in the solid state relay Icz flashes, causing
the light bulb connected in series at the output terminal K to flash until IC1a. , until the monostable time of IC1b ends or the doorbell button is released before then
. The light-emitting diode VD3 flashes synchronously with the light-emitting diode in IC
2 , indicating that the circuit is working normally. The power of a driven light bulb can reach up to 100 W.