Dimming and timing dual-purpose desk lamp circuit

The desk lamp circuit introduced in this example
is small but has stepless
dimming, and also has
a scheduled light-off function.
Its circuit is shown in Figure 53. It is composed
of two parts: a thyristor lighting circuit
and a transistor delay circuit
.
    The dimming circuit consists of a relaxation oscillator composed of a single-junction transistor vU and a thyristor VT1. The barrel potentiometer
RPI can change C. The charging rate changes the oscillation frequency of the relaxation oscillator, causing
the arrival time of the trigger pulse of the gate of the thyristor VT1 to change. The conduction angle of the trapped VT1 also changes accordingly, thereby achieving
the purpose of stepless dimming of the bulb E. The timing circuit is composed of transistors VT2, VT3, etc. After turning on the switch S2
, the timing circuit starts to work. The timing working time of the desk lamp F is controlled by the potentiometer RP2. The power supply charges the capacitor G through
VT3. When the voltage across Cz reaches the sum of the forward voltage drop of VD5~VD7 and the emitter junction
voltage drop of VT2 (about 28V), VT2 is turned on, C. If it is short-circuited, the relaxation oscillator stops vibrating, and the VT1 gate cannot
get the trigger pulse. When the alternating current crosses zero, it turns off and the bulb E goes out. If switch S2 is closed. VT2
will always remain in the off state. The existence of VT2 will not affect the operation of the dimming circuit, so the timing circuit will not work
. Using the graphic data, the timing time of this circuit can be continuously adjusted between 3min-1. 5h. The voltage regulation range of the voltage at both ends of bulb E is 10~210V.     During production, switch Sl can be linked with RP1, and sz is a separate small l×i toggle switch for ease of use.