During the DvdRA, we received from Hans Boonstra, a satisfied customer, the schedule of an auto power on / off circuit for the bat detector. If you add this, the bat detector automatically switches when you connect a headset.

1) Remove original diode
2) replaced by solid state switch
3) Connect R2 to unused connector headset connector
4) Cl join a mass point

By connecting a low-grade stereo headphone or any mono headphone, Q1 is guided and the bat detector turned on. The combination of R2 and C1 prevents audio signals from affecting the switch.

Partlist

R1 100k

R2 470

C1 10U

Q1 BS250

D3 ln4148

In the Dutch building descriptions you can also download the original PDF

Thank you Hans