Only 2 transistors OR

[Gervm]

Circuit was printed in very old Russian magazine many years ago. This devise can find aluminium disk with diameter 150mm at the depth 0,8m. When I was young I made several units. They worked good but this circuit must be made with exactness as any OR. If you stabilize suply it will be more stable.

[Gervm]

Oh, lost one connection - collector of T1 must be connected to C2. Now is all right.

[Geo]

Before many many years i made one similar from an Italian schematic with the crystal filter at 1Mhz. It was not so sensitive as say for it!!!
Did you tried it at higher frequencies (350... 500Khz)???

[Gervm]

I saw and made circuit you say. I think sensitivity depends on Q-factor of the cristal you use and working point you choose. You may also use LC filter but result equel - higher Q and your sensitivity will be higher. I can explain it on picture

[Geo]

I saw and made circuit you say. I think sensitivity depends on Q-factor of the cristal you use and working point you choose. You may also use LC filter but result equel - higher Q and your sensitivity will be higher. I can explain it on picture

You have right.
The crystal filter at 1Mhz had a bandwidth of 200.... 300 Hz. Very good.
I have some crystals at 380Khz, so maybe to give a try at your schematic.... (i don't have crystal at lower frequencies)..

[Gervm]

I think it will be work but than higher frequency than more capricious device. Also you`ll have to decrease number of turns L1 and value of capasitors.

[Gervm]

You may buit OR detector without cristal. For example this circuit. Sensitivity depends on Q of LC filtre.

[Geo]

No.... there is a big difference at Q between xtal filter and LC Filter....

[Gervm]

You are right but LC filter you can make by your hands - cristal no...

[Gervm]

One more idea for OR - you may connect an audio output for this sort of detectors.