Timing Control in the Delay Circuit
Output Q1 of this delay circuit becomes active once the switch-on period has elapsed and remains active until the next cycle begins. In contrast, output Q2 functions as a monostable after its mono period has elapsed and automatically becomes inactive. The duration of the mono period can be adjusted between 1 second and 4 seconds using the variable resistor P2. Additionally, the power-on delay can be precisely set between 1 second and 1 minute using the potentiometer P1 and the DIP switch S1.
Count-Down Indicator and Display
The circuit is equipped with a count-down indicator. The darkened display on LD1 provides continuous information about the remaining switch-on time. Driving the 7-segment display is IC4, a decoder chip that selectively shows only the normal digits (1–9), ensuring a clear and accurate representation of the remaining time.
Managing the Delay Period and Output Activation
When the delay period concludes, counter IC1 reaches position 0, causing the output of IC3a to go high, activating Ti. Simultaneously, the output of IC3b toggles from high to low, triggering monostable IC2d. Consequently, the monostable’s output goes high, illuminating the decimal point of the display and turning on T2. Additionally, the start/stop bistable IC2b-IC2c is reset, disabling both the counter and the oscillator. To initiate the next time-out cycle, the bistable must be freshly set. This requires the last transition (trailing edge) at the input, which can be achieved by pressing S2 or through another circuit or sensor while link JP’ is in the specified diagram position.
Alternate Operation Modes and Power Requirements
When JP’ is in the alternative position, the output of the monostable is connected to the input of the start/stop bistable (flip-flop). This setup causes the time-out cycle to restart at the end of the mono time, effectively transforming the circuit into an oscillator. The circuit operates within a power supply range of 8–15 V and consumes approximately 40 mA, with the majority of the current drawn by the display. Transistors T1 and T2 have the capacity to switch loads up to 400 mA.