Hi all:
I'm building a simple 555-based timer circuit to control a relay. It is a monostable, negative edge triggered circuit, so that the relay is energized for up to two minutes but if the switch is held low past the two minutes, the 555 output falls, and the relay lets go. I'm using pin 4 (Reset) to stop timing if the switch is not held past the two minute critical time. I am using a protection diode, and another diode from the V+ to the coil, then a transistor to drain the coil to ground based on the 555 output into the base.
The circuit was working fine, I thought, so I wired up the main load, which is a peltier cooler for a bar refrigerator, and now it works sometimes. When it fails, the relay clicks for the briefest moment at the critical time (2 minutes, based on R1 = 470Ω and C1 = 220µF) but I guess it's getting retriggered. So there is noise in it somewhere, but I thought I did everything to suppress that, including the relay protection diodes, a small capacitor at the control (pin 5), and a smoothing capacitor (100uF) between V+ and ground.
Since I don't have an oscilloscope, I don't know for sure that the fault is happening at the trigger input, or somewhere else wierd.
I'm building a simple 555-based timer circuit to control a relay. It is a monostable, negative edge triggered circuit, so that the relay is energized for up to two minutes but if the switch is held low past the two minutes, the 555 output falls, and the relay lets go. I'm using pin 4 (Reset) to stop timing if the switch is not held past the two minute critical time. I am using a protection diode, and another diode from the V+ to the coil, then a transistor to drain the coil to ground based on the 555 output into the base.
The circuit was working fine, I thought, so I wired up the main load, which is a peltier cooler for a bar refrigerator, and now it works sometimes. When it fails, the relay clicks for the briefest moment at the critical time (2 minutes, based on R1 = 470Ω and C1 = 220µF) but I guess it's getting retriggered. So there is noise in it somewhere, but I thought I did everything to suppress that, including the relay protection diodes, a small capacitor at the control (pin 5), and a smoothing capacitor (100uF) between V+ and ground.
Since I don't have an oscilloscope, I don't know for sure that the fault is happening at the trigger input, or somewhere else wierd.