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I’ve built a circuit. Here’s the schemic:
The values used allowed me to have intervals from about a half-second all the way up to nearly a minute. The drive or "high" time controlled by the monostable timer (R3 and C3) allows for timings between a few milliseconds to a few seconds.
I added R5, a 1K resistor, later to ensure that the interval wouldn't go below a few hundred ms.
It's been running quite well, actuating the solenoid at regular intervals as designed. The TIP122 transistor is barely warm, the solenoid is within normal temperature for its duty cycle, but the 556 timer IC is too hot to touch. It's been operating continuously for 24+ hours without any failures, but I am concerned because I think that this IC should not be warm at all.
I’ve read the details of 556(http://www.kynix.com/Detail/1126840/NE556.html), and still have some questions. Is there something I overlooked in this schematic that should be done to prevent excess heat in the IC? I haven't (yet) measured current usage by the IC alone, but the total circuit draw is 125 mA on standby and 2A when the solenoid is actuated.