MAGENTA ELECTRONICS
BAT DETECTOR MKII
Designed by Mark Stuart
Introduction
This version of the Magenta Bat detectors was
designed to be easily assembled by electronics hobbyists, low cost, and reliable. It is a 'heterodyne'
circuit which mixes a variable frequency oscillator
with the received ultrasound to produce an audible
output. It covers the range from 20kHz up to 140kHz
and is suitable for group use, as it has a powerful
audio output stage that can drive up to 0.5 Watts into
a 16 ohm load. A standard 3.5mm jack socket can be
added for personal stereo headphones or connecting
to a tape recorder. The circuit uses a high performance balanced mixer which minimises noise and interference and operates at low current levels.
Circuit Description
The circuit diagram is shown in Fig. 1. TR1
and TR2 form a two stage ultrasonic amplifier. The
capacitor values C1, 2, and 3 have been chosen so
that the amplifier gain falls off steeply below 20kHz
to cut off any audio frequency sounds that may be
picked up.
From the amplifier the signal passes to the
mixer circuit IC1 where it is combined with the signal
from oscillator IC2. The frequency of oscillation is
set by the charging and discharging of C10 via R8
and RV1. R8 sets the high frequency limit to 120kHz
when RV1 is set to minimum, and when RV1 is set to
maximum the low frequency limit of 20kHz is produced. The output from IC2 is initially a square
wave, but this is filtered to remove harmonics and
lowered in amplitude to a suitable level by R7, C7,
and C8.
The output from the mixer consists of two
signals - one is the sum of the two input frequencies,
and the other is the difference. The sum is always
above the audible range and is not wanted, whilst the
difference is the wanted 'heterodyne' signal which
contains all the characteristics of the received ultrasound but shifted down in frequency. If a simple
mixer had been used, the output signal would have
also contained the oscillator signal and the original
ultrasound signal. A 'double balanced mixer' of the
type used in this circuit cancels out these two unwanted signals.
The output from the mixer is passed via R10,
C9, and C11 to audio amplifier IC3. The gain of IC3
is controlled by RV2 which varies the DC voltage
level applied to pin 4. This type of control is becoming more popular as it allows a simple linear variable
resistor to be used, and allows the audio signal to be
connected directly to the IC - avoiding possible interference pick up.
Amplifier IC3 is a 'full bridge' type containing
two output amplifiers which operate in opposite
phase driving one end each of the speaker. This
arrangement is particularly effective where low voltage supplies are used as it allows twice the output
voltage swing to be obtained. It also means that there
is no need for an output coupling capacitor. Another
useful feature of IC1 is that it has internal 'automatic
shut down' protection from short circuits and overheating.
Assembly
The circuit is built on a single printed circuit
board (pcb) which is mounted by means of four
screws and spacers on to fixing posts moulded into
the case. A separate small board is used to mount the
ultrasound microphone.
Loudspeaker
The loudspeaker is a special low profile type
with a waterproof mylar cone. It is necessary to
remove the centre pcb mounting pillar from the inside
of the case front so that the speaker can fit correctly.
The pillar can be removed easily with sharp wire
cutters, or can be drilled out with an oversize drill.
Make sure it is removed completely so that there is
nothing left that might touch the speaker cone and
cause ’buzzing’ sounds. Fit the speaker using a solvent based ‘household’ adhesive. Make sure the adhesive goes all the way round the black mounting
face but does not touch the speaker cone and fit the
speaker with the two solder tags facing towards the
battery compartment. Attach one sticky pad to the top
of the speaker magnet as a spacer to make sure the
bottom of the pcb does not contact the speaker.
Microphone mounting
The microphone is mounted (either way
round) on a separate small board. Make the connections to the board before fitting it into the case. The
two copper connecting pads should be closer to the
LED hole in the case. The print on the microphone
side of the board shows which way round to connect
the screened lead to the rectangular undrilled pads on
the rear of the board. Take care to connect the screen
to the correct point on both pcbs otherwise the lead
will behave more like an antenna and strong radio
stations will be picked up! Fix the microphone board
to the front end of the case using two layers of
resilient ‘sticky pads’.Take care to centre the microphone in the hole so that it does not touch the case.
If a headphone socket is to be used, a single
6mm hole should be drilled in the side of the case
nearer RV2. It is a good idea to do this last - to make
sure the socket doesn’t get in the way of the main
board or other components.
PCB Assembly
Use the parts list, the component layout diagram Fig. 2, and the printed layout on the pcb. Fit all
of the resistors, and use one offcut resistor lead in the
position marked ‘LINK’.
Next fit the IC sockets. (note that the IC
sockets have one end different from the other to
indicate which way the IC should be inserted. Note
also that IC1 fits the opposite way round from IC2
and IC3 - see the notches shown in Fig. 2.)
Make sure all 8 of the IC socket pins go through the
board as they can be inclined to fold invisibly under
the socket moulding.
Fit the transistors so that their outline shape matches
Fig. 2 and finally the capacitors. Note that only C4
and C12 have their polarity marked - usually by a
band of ' - ' signs adjacent to their negative leads.
These two must be mounted as shown with their
positive leads next to the ' + ' signs on the layout
drawing. The other capacitors are non polar, so can
be fitted either way round.
The two controls, RV1, and 2 are mounted
from the component side so that their spindles pass
through the board to the copper track side. RV2 is the
volume control and also has an on/off switch. Fit the
mounting washers and nuts and make sure that the
connecting tags fit correctly. One twisted pair of
wires should be fitted for the speaker, and one for the
headphone socket, if it is to be fitted. The headphone
socket is connected with one wire to the 'common' or
sleeve terminal which is nearer to the threaded fixing,
and the other wire to both the ring and tip terminals
at the rear of the socket. Inspection of the socket will
show which connection is which.
Fig 2 shows the headphones and loudspeaker
connected in parallel so that they work together. This
works well as the headphones are normally used at
low volume levels that are nearly inaudible in the
speaker. If required, a miniature slide switch can be
added to allow the speaker to be muted when headphones are used.
The battery clip is connected with the black
(negative) wire to the point marked ' 9V BAT NEG'
on the board, and the red (positive) wire to one of the
tags on the on/off switch indicated as ‘B’ ‘9V BAT
POS’. The other tag ‘A’ on the on/off switch must be
connected with a short length of insulated wire to the
adjacent point indicated on the board.
Indicator light
A low current l.e.d. indicates when the circuit
is switched on, provides a useful indication that the
battery is not flat, and acts as a reminder to switch off
after use. The l.e.d. is push fit into the 3mm diameter
hole in the case, and should be mounted on the board
to allow some flexibility in the leads as shown in the
picture. The longer lead is the positive and is nearer
the top edge of the board. Leave the leads unsoldered
until assembly is completed and the led can be positioned accurately. Solder
ONLY to the two pads on
the left side of the picture.
This will allow the loops
in the led wires to be manipulated to adjust the led
position.
Testing
When assembly is complete and when the
board has been inspected for dry joints, solder bridges, and incorrect components, the circuit can be
tested. Start with all ICs removed so that any gross
faults such as solder bridges show up before they can
damage anything. It is useful to be able to measure
the circuit current drain whilst testing. and to have a
simple current limiting device in circuit to limit fault
current. A 10 ohm resistor in series with the battery
is a very effective way of saving components from
distress. Complete the testing with the board loosely
in position. Don’t fit the mounting screws and pillars
until everything is working correctly.
Without the ICs, switch on and check that the
current drain is around 5mA. Check that the l.e.d. Is
lighting , and that the on/off switch works correctly.
Switch off, fit IC3 and there should be slight background noise in the speaker as the current rises to 10
mA or more and the volume control is advanced.
Adding IC1 will increase the current and background
noise only slightly as there is no oscillator signal.
Finally turn down the volume control and add IC2.
The noise level should now be higher, and simple
sounds should be picked up - jangling keys, rubbing
dry hands together, and many other 'ordinary' sounds
have substantial ultrasonic components and will be
heard easily. Any resistance fitted in series with the
battery will have to be removed from this step onwards, as the extra voltage drop caused will limit the
available output and probably give rise to low frequency instability. At this stage the detector is ready
for final assembly.
Cutting the Spindles:
Before mounting the board, cut the spindles
of RV1 and RV2 to a suitable length using a fine
small hacksaw. MAKE SURE to allow for the extra
pcb mounting spacers and don’t cut the spindles too
short. Clean away all the plastic ‘bits’ before fitting
the label.
Labelling:
The self adhesive front panel label is easier to
fit if the spindles of RV1 and RV2 are not showing
through the holes. Wipe the front of the case to make
sure there are no small particles that will be trapped
under the label. Peel the label backing, remove the
‘blanks’ from the four label cut-out holes, align and
fix the label to the case using the case front recess as
a guide. The adhesive allows repositioning, but try to
avoid it if possible.
Mounting The pcb
The pcb mounts on four pillars moulded into
the case, but must be raised by spacers in order to
clear the loudspeaker. Fit the four mounting screws
through the holes in the board - note that the screws
will have to be ‘screwed’ through the board because
the holes are tight, but this will be an advantage as the
screws are retained while the spacers are fitted. This
operation is best done ‘upside down’ otherwise the
spacers fall off the screws! Position the screws in the
pillars and then turn them so that they spin in the pcb
and bite the pillars. Note that the screws are a little
oversize, but that they make good rigid mounting for
the board. Take care not to overtighten them because
the pillars are relatively soft and can be stripped or
split.
Bat Detecting!
Unless you have a tame Bat, (which is probably illegal) there is no other way of testing except to
wander out at dusk, stand, and wait. Start listening at
40kHz and medium volume. Remember that ultrasound is very directional so move the detector to scan
the area. Jangling keys will produce plenty of ultrasound for functional tests.
Battery.
The battery life depends upon the volume
level. Most of the power drain will be that used by
IC3 driving the speaker. The circuit will give reasonable life from a standard PP3, but an alkaline type is
recommended for regular use. Modern 8.4 Volt
NiMH rechargeables are also suitable and give satisfactory life between charges.
Copyright:
This circuit and hardware design was produced by
Mark Stuart for Magenta Electronics Ltd.
Magenta Electronics Ltd,
Unit 1 Farmer Court,
16-17 High Street
Tutbury
Staffs. DE13 9LP U.K..
[email protected]
R1,4,11,12
R2, 10, 15
R3, 8
R5
R6
R7
R9,13
R14
RV1
RV2
220k
4k7
10k
2k2
100R
22k
470k
220R
100k LIN
100k LIN Sw.
0.25W 5% Carbon Film
Resistors
TR1, 2
IC1
IC2
IC3
LED
MIC
LS
SK1
BC184 NPN
SAA602 Double Bal. Mixer
TL071 Op Amp
TDA7052A Audio Amp
3mm low current l.e.d.
M10-227 Ultrasonic Mic.
16 Ohm Speaker
3.5 mm Socket
Min. Pot
Min. Pot. + SPST Switch
C1, 2, 3, 7
C4
C5*
C6, 11
C8,9
C10
C12
1nF 50V 10%
100uF 10V
- Not Used
10nF 50V 20%
47nF 50V 20%
470pF 50V 10%
220uF 16V
Film type 5mm lead pitch
Rad. Elect. 6mm dia
Ceramic 5mm lead pitch
Ceramic 5mm lead pitch
Ceramic2.5/5mm lead pitch
Rad Elect. 10mm body dia.
Battery Clip, Case, Label, 2 Knobs,
Main pcb, Mic pcb, Screened lead 8cm,
7/0.2 con. wire 6 colours 15cm each,
5 Small square sticky pads,
4 No 4 x 13mm Screw,
4 x 5mm plastic spacers
Magenta Electronics Ltd. Unit 1 Farmer Court 16-17 High Street Tutbury Staffs DE13 9LP
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