G8MNY > TECH 03.09.25 08:30z 114 Lines 5999 Bytes #187 (0) @ WW
BID : 40106_GB7CIP
Read: GUEST
Subj: Car Dwell Angle & RPM Meter
Path: ED1ZAC<ED1ZAC<IZ3LSV<IK6IHL<IK7NXU<HB9ON<DK0WUE<PD0LPM<VE3CGR<VK2RZ<
VK2IO<GB7BED<GB7JED<GB7CIP
Sent: 250903/0821Z @:GB7CIP.#32.GBR.EURO #:40106 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Jan 09)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
This old device (from an old mag article) is only suitable for petrol cars with
standard distributor contact breakers. Just 3 leads are needed 12V, 0V, & the
Contact Breaker. It give 4 ranges 0-20 DC, 0-2k RPM, 2-6k RPM, & Dwell time
(90-0 4cyl & 60-0 6cyl), although it could easily be calibrated for any range.
It uses a reg & 2 74 ICs, a Hex inverter & a monostable timer.
+12V 1N4001 ____
>ÄÂÄ33RÄ´>ÃÄÄÂÄ´7805ÃÄÂÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄ>+5V
³ +³ ÀÄÂÄÄÙ ³ 560R R 27K ÚÄÄÄÁÄÄÄ¿ 470
33K === ³ === _³_ Red ÃÄÄ´11 14 ³ ³
³Cal 100u³ ³ u1³ \_/=LED C === ³ ³ 1N4148 ³
3K3 ÁÄÄÄÁÄÄÄÄÁÄÄÄÁÄÄ0V u22ÀÄÄ´10 6ÃÄ´>ÃÄÄÂÄÄÄÄÄÄ¿ ³
³ Volts to ³\ ³ 74121 ³ ³ ³ ³
À>Meter SW+ +5V 3³ \ 4 ³ MONO ³ 5K 5K 200R
DiodesÚ´>ÃÄÄ´ ÚÄ´7405>oÄÄ´5 ³ Ú>2000 Ú>6000 Ú>6000
1N4148³ ³\³14 ³ ³ / ÚÄ´4 ³ ³ RPM ³ RPM ³ Zero
CB Ú ³ 1³ \ ³ ³/ ÃÄ´3 7 ³ Meter³ Meter³ Meter³
>Ä3K3ÄÂÄ´<ÃÄ3K3ÄÂÄÄÂÁÄÂÄ´7405>oÄ´ ³ ÀÄÄÄÂÄÄÄÙ SW+ ³ SW+³ SW- ³
0.5W ³ 5V ³ 3K3_³_³ / 2³ 0VÄÁÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÄÙ
=== === ³ /_\³/³7 ³ ³\ ³\ ³\
47n³ 68n³ ³ ³ ³ ³ ³ \ 11³ \ 10 5³ \ 6
>ÄÄÂÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÁÄÄÁÄÄÄ´ ÀÄÄ´7405>oÄÄÄ´7405>oÄÄÄÂÄÄ´7405>oÄ¿
-ve³ INPUT AMP 0V 13³ / 12 ³ / ³ ³ / ³
³ ____ ³/ ³/ /³ ³ ³/ 2K7
³ -/.6mA\+ / ³9 ³ ³
ÃÄo<´Meter ÃÂÄ>o< 0-20V Volts ÚÄÄo<7405ÃÄÄÄ´ 5K<ÄÄMeter
6k-)Äo ³ \____/ ³ o< 2000-6000 RPM 68R 8 \ ³ 27R DWELL SW+
ÃÄo ³ + ³ o< 0-2000 RPM _³_ \³ _³_ ³
ÀÄo ÀÄÄÄ´ÃÄÄÄÙ o< Dwell Open\_/=yel Close\_/=gn ³
220u 0VÄÁÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÙ
HOW IT WORKS
The 12V is regulated to 5V for the logic to maintain meter calibration (with a
different regulator a 6V version could made.) A calibrated meter supply
resistor (33K + 3K3 for my meter!) gives the voltage feed to the meter switch.
³ Coil
Coil 300VÞ Resonance The high voltage spiky contact
Voltage PrimaryÞ breaker input is filtered,
12V___ back EMF³Ý_____ offset voltage applied
³ pulse ³² (for 12V) with the series 5V
Open ³__________________³± Open zener the 3 Rs, refiltered,
OV Closed Ý then clipped before being
DWELL (ON) TIME ³ buffered by the 1st hex amp.
Spark
Buffer This gives a faithful
Output __________________+5V inverted square wave output
______³ ³______ that represents the actual
contact state.
This is re-inverted & fed to trigger the monostable IC to remove the on - to
off ratio. The monostable's CR time is set to be a bit faster than the fastest
6000 RPM (10mS), so the resultant square wave output's on to off time varies
with the engine revs.
ÚÄ¿ ÚÄ¿ ÚÄ¿ ÚÄ¿ ÚÄ¿ ÚÄ¿ ÚÄ¿ ÚÄ¿ ÚÄ¿ÚÄ¿
Mono ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³³ ³
OutputÙ ÀÄÄÄÄÄÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÙ ÀÄÄÄÄÄÙ ÀÄÄÄÄÙ ÀÄÄÄÙ ÀÄÄÙ ÀÄÙ ÀÙ À
Low Revs High Revs
Mean _____.......----~~~
DC _________________............--------ÄÄÄÄÄÄÄÄÄ~~~~~~~
This is the fed via a diode to lose any IC "off" DC, to the meter via 2 presets
one with a -ve meter offset to give the 2 linear RPM scales. The meter is
heavily damped with a 220uF, to give a steady reading at low Revs.
The dwell angle feed, uses up the rest of the Hex amps to further buffer the
signal & drive 2 LEDs to show the contact status, & also the dwell meter feed
via a calibration pot.
Coil With contact breaker ignition it
Current _..--ÄÄÄÄÄÄÄÄÄÄÄ¿ is important not to have to short
0A___.Ä~ ^ 5Amp ³____ a Dwell time as the coil takes time
|Charge| to build up to full current &
OFF | DWELL ON | OFF "charge up" its magnetic circuit.
Typical recommended 4 cylinder values are 33-45% ON times depending on the
maximum engine RPM & ignition coil type. So called "Sports" types are really 6V
coils & an external 100W resistor to safely limit the stationary current (10A!)
to give faster charge times for higher revs, then came along electronic
ignition with its 300V via SCR to coil DC pulses. :-)
METER SCALES
The meter scales are quite linear, the Dwell scales are reversed & do not start
at 100% !
Deflection Volts 2K RPM 2-6K RPM DWELL 4Cy DWELL 6Cy
0% 0 0 2000 90% 60%
25% 5 500 3000 67«% 45%
50% 10 1000 4000 45% 30%
75% 15 1500 5000 22«% 15%
100% 20 2000 6000 0% 0%
Why there are different Dwell scales for 4 & 6 cylinders I do not know!
CALIBRATION
Using a square wave astable source like 555 with a frequency counter & ordinary
analogue meter you can mimic the engine pulse on the workbench while varying
the frequency & on to off ratio. Using just a low voltage 50Hz as the input you
can only calibrate 3000 RPM it is a good enough if it is all working.
Why don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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