This device (Fig.1) automatically disconnects the load (player, CD player, amplifier, etc.) at the end of the phonogram or CD-ROM with the loss of signal at the input LF XS1.
Fig.1
When you press the SB1 button the voltage is supplied to the transformer T1 (Fig.2). Rectified voltage power supply +15 V is supplied to the device.
Fig.2
Simultaneously with the closure of the contacts SB1.1 is charging the capacitor C4 through the resistor R9. The inputs 5 and 6 of the element DD1.2 is a logical "1", the output 11 element DD1.3 is also a logical "1". This leads to the triggering a transistor switch VT1 VT2...and tripping the relay K1, which contacts K1.1 and K1.2 blocks contacts SB1.2 and SB1.3. Now when you release the button SB1 the device remains connected to the network, since the capacitor C4 is charged and holds a logical "1" at the inputs of the element DD1.2.
This condition until while C4 is discharged through the resistor R10. If "log LF" signal with the line output of a tape recorder or other radio equipment, the comparator DA1 fire even from very weak LF signal, at its output appears logic "0" respectively at the exit of the element DD1.1 appears logical "1", which charges the capacitor C4. The led VD3 is used as an indicator of work of the comparator.
When a signal of the input ХS1 the capacitor C4 is discharged through R10, and when these values of C4 and R10 after about 2 minutes on the inputs 5 and 6 of the element DD1.2 will be a logical "0" that will turn off a transistor switch and de-energize the device.
The setting is in the selection or R10 C4 for the necessary delay the off time.
Details. The transformer T1 - for a current of 0.1 A and a voltage on the secondary winding 12...15 V. the Diodes VD7...VD10 - CD or D226, you can use a diode bridge to the corresponding current and voltage, the Chip CSS can be replaced by CLA. SB1 - P2K with lock or without lock, if you do not need to rewind the tape and etc. While pressing the SB1, the load will be on all the time.
Literature
Author: V. Dube, p. Shumilino, Vitebsk region