-The circuits page
On this page I will feature some useful
circuits that I've used over the years.
. .50A trip . .136 preamp . .LF loop preamp . .LF preselctor..50A keyer.
Audio level control device.
Starting with an audio compressor ( well
you've got to start somewhere! ). There have been umpteen designs for
compressors published over the years but many of them are difficult to set up
or cause distortion and others use expensive chips.
This one updates an old principal I used
years ago: a light bulb driven by audio illuminates an LDR in the feedback
loop. The brighter the light tends to glow, the lower the resistance of the LDR
becomes and the more negative feedback gets applied, reducing the gain. The LDR
is purely resistive so the amplifier maintains good linearity.
The problem with a light bulb is its' thermal
inertia, the attack time of the AGC circuit is too slow and you get distortion
on peaks. What we needed was a fast light bulb.... We now have one, the light
The circuit is shown above and should cost
about £10 if you have to buy all the bits new.
The NORP12 L.D.R has a dark resistance of
around 1MegOhm but will come down to a few hundred Ohms in bright light. It's
peak sensitivity is to yellowish light at a wavelength of around 580nM. The
best match is, of course, a yellow LED. Toshiba have some nice bright ones with
a peak output at 590nM which is close enough!
The crux of the thing is to mount the LDR and
LED in close optical contact whilst being careful to exclude all stray light
from the assembly. I mounted mine in a piece of black plastic tubing and sealed
it with silicone rubber and black end-caps.
The op-amps are two halves of a TL072 and
have enough gain to bring a mike insert up to a good loud line level.
Performance has been measured as below.
Performance with gain set to
|theshold of compression
|maximum input level
40dB input range is compressed to 3dB output
Distortion is less than 0.2% within normal
Frequency response is 30Hz to 8kHz within 3dB
at max gain.
The current drawn at 12V is only about
|Farnell part number
|Cost ( ex VAT and carriage )
|Op amp Chip TL072
|Light dependant resistor NORP12
|Toshiba yellow ultra-high-bright
The resistors are nothing special, 0.4W metal
film are OK; the diodes are small signal diodes such as 1N4148 and the
capacitors are at least 12V working.
Do not attempt to operate the circuit below
12V as there won't be enough drive to the LED and clipping will result. Using
germanium diodes instead of silicon ones would give a little more headroom (
and slightly reduce the output level ).
Take a high-current trip.
This one is useful if you are experimenting
with high current circuits (like mosfet PAs!). It is a trip which will quickly
break the circuit when the current exceeds about 50A. The pass transistor is a
power fet with an on-resistance of 0.007 Ohms. This means that at 50A it will
drop about 0.35V, just enough to turn a germanium transistor on.... Yes that's
the snag, you need a good junkbox with some OC140s or something similar in it!
As the transistor starts to conduct, it will reduce the gate Voltage on the fet
and begin to turn it off, this causes the Volt drop to increase and it
avalanches itself into the tripped state with the LED glowing merrily. Pressing
the reset button shorts out the base bias of the transistor allowing the fet to
turn on again. Only press this when you're sure you know why it tripped in the
The 9V battery will last for ages in the
untripped state because the only current drawn is the leakage of the transistor
which should be a few microamps.
|Cost ( ex VAT and carriage )
|£5.30 + VAT
|Ge NPN transistor
|see list below
|free from your junk box?
If you want it to trip at a lower current you
can add a bit of resistance at X, it's best to do this by trial and error, just
a few inches of reasonably thick wire should do. Do not use it at more than 12V
or so, or the current through the 4k7 resistor will exceed the base current
rating of the transistor when tripped.
It's all a bit experimental but it works
pretty well, you made need to add some Cs here and there to prevent unwanted
Possible germanium NPN
- NKT713 to NKT781
Have a good root through your tat box and you
may find one of these, it's a pretty comprehensive list! I'm not guaranteeing
that all of these will work, most of them will be thirty years old and may be a
bit leaky.... best to check 'em first. I used an OC140 in mine.
Pre-amp / filter for 136kHz.
Most modern tranceivers cover LF but the
performance is not always very good. All you can hear are strange whistles and
burbles which aren't really there! You need this little circuit. It has a gain
of about 10dB and a nice sharp band-pass response about 3kHz wide, enough to
cover the 136 band.
The coils are Toko type 719VXA-A017A0 and are
available from BEC on 01753 549 502 at about 90p each. If they are out of stock
try type 719VXA-A018AO which has a slightly lower inductance. Note the
More gain can be had by connecting the fet
gate to the top of the second tuned-circuit, instead of the overwind. Done this
way, you could use any variable inductor with an inductance of around
Loop Pre-amp for LF.
If you want to use a a small multi-turn loop
on LF, you will find that the sensitivity is not good enough to receive the
weaker signals. A good preamp for a loop needs to have a high input impedance
and good signal handling characteristics (to prevent cross-modulation). The
circuit shown below is the best I tried and has enabled me to hear Italian
amateur signals on the 136kHz band with a 1m diameter tuned loop. The preamp is
meant to be mounted outside, near the loop, and is powered down the coax from a
12V supply in the shack. The +12V is applied via another choke at the RX end. A
blocking capacitor is also required to prevent 12V being applied to the RX
The loop I used for 136kHz had about 30turns
of 22g wire wound round a 1mtr square former and tuned with a 1000pF variable
C. The preamp is fed from a two-turn coupling loop just inside the main coil.
The preamp also seems to work quite well with a tuned whip aerial.
A preselector for LF.
Here is a preselector circuit designed by
Wolf DL4YHF as part of his transverter. It uses two pot-core filters with FETs
and feedback. By adjusting the controls, the bandwith can be set to less than
200Hz. This is the preamp which Wolf uses at the club station DF0WD.
Keyed PSU for class D
Keying the drive to a class-D mosfet PA can
create key-clicks due to the non-linear characteristic of the PA. A nice clean
keying shape can be achieved by keying the supply with appropriate rise and
fall times. In this design the series regulator is keyed thus "killing two
birds with one stone", regulating and smoothing the supply and keying the PA.
Two seperate rectifiers and smoothing Cs are combined to give adequate
The power-fets need to be mounted on a large
heatsink and may need force-cooling. I mounted the 40A bridge rectifiers on the
heatsink too. This is big stuff, engineer it accordingly.
I'll try and add some more useful circuits
over the next few months. If you have any that you think should be seen on
here, e-mail me!
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