AUDIO
OUT
AUDIO OUT
L
R
By Jake Rothman
PE Mini-organ – Part 2
R34, R35
12kΩ
R36, R37
3kΩ
R38
15kΩ
R39
Not used.
R40 to R63 on the board are positions
for possible trimming parallel resistors,
generally not needed.
R64 to R69
Not used
R70
1MΩ
Potentiometers
VR1 10kΩ linear – tuning
VR2§ 220kΩ anti-logarithmic – vibrato
speed
VR3 100kΩ logarithmic – vibrato depth
VR4§ 4.7kΩ anti-logarithmic – volume
(Note: anti-logarithmic potentiometers
are available from Tayda or the PE special kit.)
The PE Mini-organ
L
ast month, I introduced the
PE Mini-organ, an easy-to-build,
all-analogue, (mostly) throughhole-component design based around a
555 time chip and a PCB keyboard (see
above)... oh, and a lot of resistors!
In Part 1 we covered the design and
this month we will run through components and assembly.
Components
Semiconductors
D1, D2 SB40 1A 40V or 1N5817 Schottky
rectifier diode
D3
1N4148 standard small-signal
IC1
7555 CMOS timer IC
TR1
BC549C high-H fe NPN smallsignal audio transistor
TR2
FDC634P or ZVP2106A lowthreshold voltage P-channel
MOSFET
Resistors
All standard 0.25W size. Those marked ‘*’
are 5% carbon film. All others, especially
48
keyboard resistors R14 to R38, must be
1% metal-film.
R1
R2
R3
R4
R5
R6
R7, R11
R8, R12*
R9
R10
R13
R14
R15, R16
R17, R18
R19, R20
R21
R22, R23, R24
R25
R26, R27
R28
R29, R30
R31, R32
R33
68kΩ*
22kΩ*
4.7MΩ*
30kΩ*
12kΩ*
100kΩ*
47kΩ*
2.2kΩ
330kΩ*
2.2MΩ*
Not used
62kΩ
3.9kΩ
4.3kΩ
5.1kΩ
5.6k
6.2kΩ
6.8kΩ
7.5kΩ
8.2kΩ
9.1kΩ
10kΩ
11kΩ
Capacitors
C1, C2, C3 150nF 5mm box polyester
C4, C7, C11 470nF 5mm box polyester
C5§, C6§, C8§, C10§ 10nF 5% tolerance
or better, C0G/NP0 ceramic, polystyrene
or polypropylene. The PCB will
accommodate axial or radial types. These
are available in a special kit.
C9
470µF 16V radial electrolytic
C12
1nF ceramic 10%
C13
470pF ceramic 10%
SW1 Miniature PCB-mounting SPDT
toggle switch Rapid 75-0125
SW2 4-pole 3-way rotary switch makebefore-break PCB mounting (or
cut tag eyelets off) Lorlin or
Taiwan Alpha SR2612-0403-38FS
(Rapid 79-0215)
Connectors
2.1mm DC connector PCB mount
(Rapid 20-0970)
Solid metal uninsulated 4mm banana
socket (Rapid 17-0597)
Standard USB A socket
USB 2.0 mini B jack (Mouser 490-UJ2MBH-1-SMT)
Mono or Stereo 6.3mm PCB-mount
jack socket
Loudspeaker
50Ω to 80Ω§ 67mm minimum. NOS ITT 50Ω
5 × 3-inch speakers in special kit. These
are also available from Breconjess.co.uk
Practical Electronics | July | 2020
n
USB mini
DC connector
D1
+
R2
C10
C6
R4
C2
C12
R3
C1
Bend cap (C9)
over to board POWER
C13
R1
C4
C5
11
CB
DA
12
5
s
g
d
TR2*
C7
R11
R5
C11
R12
R10
TR2*
IC1
R8
VIBRATO SPEED
VR2
VR1
CW
CW
1
3
e
b
c
* Only use
one TR2*
PITCH
Lug
4
TR1
R7
R70
R6
R14
10
7
6
R9
C3
C9
OCTAVE 8
Output jack
D3
D2
C8
S1
9
Speaker + Speaker –
Battery – Battery +
USB
VIBRATO DEPTH
VOLUME
VR3
VR4
CW
CW
2
R
26
R
25
R
24
R
23
R
22
R
21
R
20
R
19
R
18
R
17
R
16
R
15
R
38
R
37
R
36
R
35
R
34
R
33
R
32
R
31
R
30
R
29
R
28
R
27
Resistors in boxes
for presets or
padder resistors
PROBE
These resistors are
not numbered on
the PCB
Fig.9. PE Mini-organ PCB overlay. (The keyboard section has been cropped, but it contains no components.)
Fig.10. Completed board – it’s so good looking that it’s worth making a perspex panel for it!
Practical Electronics | July | 2020
49
Fig.11. Close-up of (left) the surface-mounted FET and (right) the USB-mini socket.
Optional output transformer Xicon 42TU200-RC (Mouser) or Eagle LT726 (J Birkett’s in Lincoln: 01522 520767).
A PCB (AO-0720-01) and a kit (AO-0720-02) of the hard-to-get
parts: four 10nF 5% caps, the 50Ω speaker and the two anti-logarithmic pots are available from the PE Shop. The parts in the kit
are marked ‘§’ in the components list.
Construction
The whole point of this design is that it’s easy to make – there’s
lots of space, as shown in Fig.9 and Fig.10. The board area is
four times that of the commercial version. Solder in the usual sequence: if used, SMT parts first, (so you can get round them with
the iron), then low-profile parts such as resistors and diodes. The
chip socket and axial capacitors should be next, and then finally all the stuff that sticks out; inserting it in order of height, such
as the transistors, radial capacitors and sockets. The pots and
switches should be last.
Fig.12. Rotary octave switch mounting –
note the stop washer and position of tab.
Note that there are multiple outlines on the pots and octave
capacitors to facilitate the use of different types. If the correct
PCB mount pots cannot be obtained it is a simple matter to insert 0.15-inch Veropins and solder standard 16mm Alpha pots
to these. Remember to cut the shafts on the rotary controls to the
right length to suit the knobs and front panel used before soldering. It’s not a good idea to wield a hacksaw on a completed PCB!
(The PCB uses plated-through-holes, which can make de-soldering a trial.) Remember the board costs more than the components,
so if a component must be removed don’t worry about damaging it; cut the lead-out right against its body if necessary. Fig.11
shows close-ups of the optional SMT components (USB-mini
socket and TR2).
Be careful mounting the rotary switch (see Fig.9 and Fig.12 to
get the numerical position right). If you get it wrong it’s a pig to
unsolder. Also, watch out that you get the indentation stop washer (with its protruding tab) in the right position. If the switch just
goes round and round or moves less than three positions then
it’s in the wrong hole.
The height of the components is critical if a front panel is to fit
properly. C9 may have to be bent over as shown in the photos.
The rotary switch is the tallest and the other switch and the pots
will need extra nuts to bring them up to the same level.
Probe
The stylus probe is made with an old test meter lead that is plugged
into a banana socket soldered into the board (see Fig.13). If the
end is sharp, it is best to round it off a bit with a file. I have seen
Stylophone keyboards almost worn away by scratchy probe tips.
The boards have a bright tin coating that seems to resist abrasion.
Case
JTAG Connector Plugs Directly into PCB!!
No Header!
No Brainer!
For the moment I’ll leave the case details – but do remember to
allow space for the protruding speaker magnet.
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into a tiny footprint of pads and locating holes in your PCB, eliminating
the need for a mating header. Save Cost & Space on Every PCB!!
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Tag-Connector footprints as small as 0.02 sq. inch (0.13 sq cm)
50
Fig.13. (Left) Use a metal 4mm socket to plug in a probe lead
for the keyboard (note the nut spacing); (right) The socket can
be soldered into the board – the protruding cable terminal
(under the board) has been cut off.
Practical Electronics | July | 2020
