Found in a PE Hobbyst Handbook of 1991, this circuit was originally proposed to time photographic baths in developing rooms but can be used for other purposes. The circuit beeps at 30-second intervals.
This circuit was found in a 90s Hands-on Electronics. The transistor can be a BC548. What it does...
In the figure we show how to use an LM324 as a voltage comparator the reference voltage is...
This circuit is based on an LM3900 transconductance operational amplifier, found in 1992...