555 latch
Although it isn't normally used that way, a 555 can be used as a latch, since it has a SR latch at its heart. The Threshold (pin 6) and Trigger (pin 2) act as the Set/Reset inputs. To work around the input comparators, the Threshold "high" just needs to be > 2/3 VCC and the Trigger "low" needs to be < 1/3 VCC. No problem if you're using VCC and GND levels for the inputs.
Threshold (S) turns the latch on, and the Trigger (R) or Reset input can be used to turn the latch off. Tie whichever one you don't use (pin 2 or 4) to VCC. The pullup/pulldown values aren't critical, they're just there to keep the inputs from floating.
Note that you only have access to the inverted state of the latch through pins 3 and 7. The normal buffered output at pin 3 can sink or source loads of up to 200mA, and the output is high when the latch is off. The discharge output at pin 7 can sink current when the latch is on and can also be used for level translation if the pull up resistor is connected to a positive supply other than VCC.
- CAL