General Purpose Time Switch Circuit Overview:
General purpose time switch: This circuit is designed to fulfill the criteria for a cost-effective and straightforward timer. It encompasses a timing range from less than a second to several tens of hours and utilizes a mains interface based on thyristors for controlling inductive loads.
Timer and Switching Sections in the Circuit:
The circuit is essentially divided into two sections: the timer section and the switching section. The timer’s precision relies on the mains frequency, either 50 Hz or 60 Hz. Components such as R4, R3, R2, C1, DI, and D2 work together to transform the mains voltage into a clock signal for the binary ripple counter IC1 (4040). IC1, IC3, and an 8-input AND gate (IC4) together form a programmable divider. Once the predetermined time, set using jumper wires at the outputs of IC2 and IC3, has passed, the output of IC4 goes low, triggering the bistable IC2b-IC2c to toggle.
This action removes the gate drive of thyristor Thy, leading to the disconnection of the load connected to K2. To minimize mains pollution and switching noises, the load is switched during the zero crossing of the mains voltage. Additionally, the timer can be configured to switch on a load after a preprogrammed time by fitting the wire jumper ‘Y’ instead of ‘Z’.
Setting the Time and Power Supply Considerations:
The time in this circuit is configured by inserting a maximum of eight wires between the 8-input AND gate and the counter outputs. The cumulative time is the sum of all the selected times specified in the table. To power the circuit, a 12-V supply voltage is derived from the mains through a phase rectifier, D3. This supply voltage undergoes stabilization and smoothing facilitated by zener diode D5 and reservoir capacitor C6. It’s crucial to note that the series resistors in the rectifier, namely R16 and R17, should not be replaced by a single 100-kΩ resistor. This is because the total voltage drop across these resistors, at a mains voltage of 240 V, is approximately 220 V, which exceeds the capacity of a single 0.5 W resistor. The same caution applies to resistors R3 and R4.
Timer Operation and Electronic Control Features:
The timer is initiated by pressing push-button S2. Activation of the timer corresponds to either switching the load on or off, depending on whether jumper Y or Z is connected. During the timing cycle, pressing S2 resets the circuit, initiating a complete timing cycle regardless of when the switch was initially pressed. The timing cycle can be halted by pressing S1. Electronic control of start and stop functions is facilitated by two optocouplers, IC5 and IC6. The electronic control inputs (K3 and K4) are electrically isolated from the timer and can be driven by control voltages of 5 V.
Construction and Safety Precautions:
Building the timer is straightforward, as all components are integrated into a single printed-circuit board, minimizing wiring requirements. Both the start and stop switches need to be rated at 250 V due to their exposure to mains potential. It is imperative to enclose the circuit in an ABS enclosure using non-metallic screws, ensuring proper strain relief and insulation for the mains input and output cables. A critical safety reminder: given the circuit’s exposure to dangerous voltage at multiple points, refrain from working on it when the mains is connected. Ensure that no part of the circuit is touchable during setting, adjustment, or operation.
- R1, R13, R14, R19 = 33 kΩ
- R2 = 100 kΩ
- R3, R4 = 470 kΩ
- R5-R12, R20 = 2.2 MΩ
- R15, R18 = 1 MΩ
- R16, R17 = 47 kΩ
- R21, R22 = 1 kΩ
- C1 = 1 nF
- C2, C3 = 100 nF
- C4, C5 = 4.7 nF, 400 V
- C6 = 220 pF, 63 V, radial
- C7 = 4.7 pF, 63 V, radial
- D1, D2, D6, D7 = 1N4148
- D3 = 1N4007
- D4 = not used
- D5 = zener diode, 12 V. 1.3 W
- B1 = B380C1500 (round type)
- T1 = BC547B
- Tri1 = TIC106 D
- IC1, IC3 = 4040
- IC2 = 4093
- IC4 = 4068
- IC5, IC6 = CNY65
- K1-K4 = 2-way PCB terminal block, 7.5 mm pitch
- S1, S2 = push-button switch for 250 V a.c,
- F1 = fuse, 1 A, delayed action, with holder for PCB mounting
- Enclosure, e.g., Bopla E430