Filter Characteristics
Apidictor Inductors
Transistor Characteristics
Article by Eddie Woods
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Live CD version
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Woods' Apidictor, Electronic Listening Device For Bees

Information regarding the Apidictor has been absent from this website for several years. There is constant interest in the device and little information elsewhere, so what is here is an important resource. I have located much of Dave Cushman's material and displayed it in what I think is close to the original form. I have not altered much, mainly repairing links or deleting those that link to other websites that no longer exist. The electronics enthusiast will see that Dave didn't complete this project and there are a number of gaps. If anyone is able to fill them please let me have details and I will make changes. All additional material or corrections will be easily identified with the original left intact.

Roger Patterson. 13th Dec 2012

The late Eddie Woods developed a listening device with filters that could detect the changes in sound produced by a colony before and during swarming.

More recently Rex Boys wrote a book "Listen to the Bees" which deals with the original equipment (it is the copyright of so is not reproduced here).

With the advent of digital and op-amp techniques it should be a simple matter to take this work further.

This page will analyse the original and propose some modern experiments that I hope some of you, younger, electronic design engineers will take up and build modern alternatives.

Block Diagram

Block Diagram of Apidictor

Block interface specifications of Apidictor

In order that different individuals and groups may update or repair original items of equipment I will stick as near as possible to the original specification with suggestions for modern equivalents.

Is battery positive, and chassis metalwork.

Is the nominal battery voltage of Eddie's transistorised versions mk4 & mk5. It is not critical, but if we use it then it helps compatibility of new and old "spare parts" and allows defunct parts to be replaced by up to date modules.

This negative voltage is a sub rail from the 9v battery and is heavily decoupled. It is used in the original purely in the pre-amp, but I have included it here in case it is required by any electret microphones or other active listening devices.

In the original a crystal microphone was used, but we will merely specify a high impedance voltage output which will then be compatible with the pre-amp input no matter how the audio signal is actually derived. The signal voltage would be in the region of 0.5v pk with a 2 megohm source impedance.

to filter
This is the pre amp output (at the moment guessed at) of around 1v p-p @ 10k impedance.

from filter
The filter output is capacitively coupled to a level setting amplifier.

full level & variable level
Are switch connections that are ganged with the filter switching enabling the volume control to be by-passed so that hiss listening is at full volume.

filter output
Is the resulting signal (either full volume or the set level) which is then capacitively coupled to the input of the final output amplifier.

The headphone output (thought to be 600 ohm), the same signal is also passed to the indicator.

The magic eye used in the original would have required about 150v - 180v on it's anode to function, this was derived from a small transistor oscillator. (I will clarify this when I track down the original data). The indicator could very easily be replaced with an array of LEDs, which gets rid of the requirement for the high voltage rail.

The Original Schematic

Has been broken down into the same blocks that are represented in the block diagram. All component values are listed in the table further down the page.

Schematic of mic & pre amp for Apidictor  

These two portions of the block diagram above, have been grouped together as any new designs will treat this as one single block.

It is simply a microphone and amplifier whose output is about 1v peak to peak.

The original used a crystal mic, but I will be offering versions using moving coil and electret elements.

The previously published circuit diagrams identified the switch positions wrongly.

This has been corrected here, in addition the resistor that gives "straight through" operation in position one, has been labeled r31.

The switch wafer order has been re-arranged, but each wafer still performs it's original duty.

  Schematic of Apidictor filters & switching

MK 5
 c150µf22µf - 100µf acceptable
 c925µf22µf ok
 c10100µf63µf - 220µf acceptable
 c111µfbetween gnd & V1 cathode on Mk 5
 c13, c142N2
 c15, c1633N
 c1741N2various capacitors in parallel to make up value
 c195N5various capacitors in parallel to make up value
 c204N42 x 2N2 in parallel
 c2151N4various capacitors in parallel to make up value
 r122K7 (nominal)Adjust on test, see note xx
 r17, r18470R
 r22100R0.5 watt
 r27, r28220Ksee note yy
 r29100R1 watt
 r30, r3122K
 r3268Kcould be 100K trim pot see note zz
 tr1, tr2OC 202OC 702
 tr3, tr4AC 107NKT 226
 tr5 - tr11OC 71NKT 224
 L37Hcentre tapped
 D1OA 85
 V1VM 160
 T1??hand wound, to be sorted
 S13 pole 4 gang
 S2push to maketoggle will be ok when the battery drain is reduced

Winding details for the chokes and transformer are available, but not to hand at the moment. Simple op amp filters will be suggested in replacement anyway.




The queenless "roar" or "moan"

Eddie's Graph

Eddie Wood's Apidictor Graph


Apidictor Panel Layout (Mk4 & Mk5)

Mk4 & Mk5 Panel Layout  

The original panels were laid out like this. With the smaller, modern components we can reduce the size considerably.

As I have done this analysis I have come to understand the device fairly well. I am prepared to help (via Email only) anyone who is experiencing difficulty in rebuilding or repairing one of these units.

this page is fragmented at the moment as it is under construction,

Printed from Dave Cushman's website Live CD version

Written Summer 2000, Revised... 01 October 2001, New Domain... 06 April 2004, Upgraded... 25 October 2005, Further Upgraded... 14 November 2007,
Source Code last updated...
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