There is another approach that can replace the 555 and its timing components with one diode. If you call a 40-second period 1 tick, then 4 hours equals 360 ticks and 6 hours equals 540 ticks. This means that 512 ticks, a nice round binary increment, equals a cycle time of 5 hours 41 minutes. If you adjust the frequency of the 4060 oscillator to 0.4 Hz (an oscillator period of 2.5 seconds), then a full cycle of Q13 equals the 5.7 hour period, Q14 going high signals the end of the delay and activates the output. Q5 has an 80 second period, 40 seconds off and 40 seconds on. Q14 ANDed with Q5 lets the valve stay on for 40 seconds, then resets the counter. This turns off the output and restarts the 5.7 hour delay period.
As in Wendy's circuit, R1 can be split between a fixed and variable resistor to tune the timing. Note that this circuit has a fixed relationship between the output on time and off time. For example, if you shorten the off time by 25% to 4 hours 16 seconds, the on time will decrease to 30 seconds.
Also, this schematic uses parts already in my design library. The caps do not need to be 50 V, the resistors do not need to be 1%, other diodes and transistors will work, etc.
ak
As in Wendy's circuit, R1 can be split between a fixed and variable resistor to tune the timing. Note that this circuit has a fixed relationship between the output on time and off time. For example, if you shorten the off time by 25% to 4 hours 16 seconds, the on time will decrease to 30 seconds.
Also, this schematic uses parts already in my design library. The caps do not need to be 50 V, the resistors do not need to be 1%, other diodes and transistors will work, etc.
ak
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