This rain, flood or leak sensor was found in a 90s Popular Electronics. All the components used are common today. The transistor can be the BC548.
This circuit was found in old documentation, but the transistor may be the modern equivalent BC547....
Found in Popular Electronics from the 90s, this circuit mutes a scanner's speaker in the absence of a...
This electromyography monitor is from a 1978 publication on operational amplifiers. The components are...