switch msw whose other side is connected by a line 25 to the positive side of the power source at 11 when the switch msw is momentarily closed a circuit is completed through lines 24 and 25 and the resistor r to the gate 23 of the scr the resistor r which in the example at hand carries a rating of 100k ohms at 1 2 watt serves to drop the 12 vdc from the connection 11 to a level which is safe for the gate 23 of the scr accordingly the latter is turned on and will conduct from its anode 20 to its cathode 21 and continue to do so as long as the current through the scr is above its holding current determined of course by the design properties of the scr in the present example the scr is a hep r1215 thyristor whose peak gate voltage is 6 vdc and whose holding current is 5 milliamperes since a circuit through the lines 12 and 13 the windings of the motor dm the lines 16 and 17 the scr and the diode d is thereby completed from the positive side of the power source to chassis ground g the negative side of the power source the scr will remain on even though the switch msw is released hence the motor dm will run and the tape play the diode d while not strictly necessary nevertheless increases stability of the circuit in that it blocks any feedback of current from the chassis ground.