This circuit was found in a Radio Electronics magazine from June 1962. The circuit is for cars of the time with platinum ignition. The indication is made on an analog instrument.
This circuit was found in an old Poptronics but is still current as transistors can be replaced...
This circuit was found in a 1960s book of projects by Gernsbak. The circuit uses germanium...
This General Electric circuit is from the 80's. The PUTs used in this meter are not common...