MFJ: RIT LED circuit


From: Paul Harden (
Date: Fri Feb 10 1995 - 22:50:54 EST

After receiving 6 requests for how I got the MFJ's to turn on an LED
when the RIT is off of zero, decided to load up the circuit and post.
The quasi-lengthy description offered for those not familiar with op
amps/comparators or for those wanting a different application.

No toroids to wind! (See notes below) R1-R3: 10K
                                               R4-R5: 100K-1 MEG
                       -------- R4 ------ R6: ~2K for +V=+12V
                       | +V | D1-D2: 1N918 or sim
        +V MFJ set | | | U1: LM158 8-pin, single
         | to +5.5v | ...8... | supply dual op amp
     10K > | : . | LED: your choice
     POT ><----- R1 ---*--3 +in . |
> : .1---*--- D1 ->[----
         | ----2 -in . |
        ///gnd | : . LM158-a |
                      | :..4... Op Amp |
                      | | |
(MFJ= +5.3v) | ///gnd *-- R6 ---
RIT V. in ---- R2 ----* | |
                      | ....... LM158-b | <=LED=>
        +V | : . Op Amp | |
         | MFJ set ----5 +in . | ///
> to +5.1v : .7---*--- D2 ->[---- gnd
><--- R3 ----*---6 -in . |
> | : . |
         | | :...... |
        ///gnd | |
                      ------- R5 --------

This circuit was built for the MFJ rigs, but could easily be applied to
anything. In the MFJ, the RIT volt. is +5.3v with no offset, and varies
+/-0.7v or so from one RIT extreme to the other. The circuit uses a low
power single supply 8-pin IC op amp, the LM158, as a window comparator.
The pot on R1 is set for +5.5v and applied to IC pin 3 and the RIT volt.
to pin 2. When RIT volt. >5.5v, pin 1 output goes from 0V to about +V.
The pot on R3 is set for +5.1v and applied to IC pin 6. When RIT volt.
less than 5.1v, output pin 7 will go from 0v to about +V. Diodes D1-D2
form an "OR" gate, such that when either op amp output goes to +V, one of
the diodes will pass the current to illuminate the LED.

None of the values shown are critical. Virtually any pot value can be
used, since its just a voltage divider to make any voltage you want
between gnd and +V. (Lower values than 10K will however draw more
current from your precious NiCads!). One could argue whether R1 and R3
are really neccessary. R4 and R5 just clamp the open loop gain of the
op amps to something so the outputs don't chatter with noise. In fact,
if you find this a problem with false triggering, place 0.1uF caps or
greater on the pot wipers. Only real caution is to make sure the input
(RIT) voltage never exceeds +V. In the case of the MFJ's, +V can be
either the +10v REF supply or the +12v. (I used the +10v).

In the MFJ, ckt draws <1mA with LED OFF; ~3mA with LED ON.
In operation, with the RIT control at the center detent position, the
LED will be off. As soon as you move the RIT control (depending upon
how tight you set the two pots) the LED will illuminate. With the LED
OFF (no RIT), the LED will illuminate when you transmit, since the RIT
is shifted 700Hz on XMIT to generate the sidetone. Since sidetone
switching comes off the key line, the RIT LED will show the actual dits
and dahs while the XMT will show the QSK setting.

This circuit can be used to monitor anything ... battery voltage, AGC,
SWR volt., anything you want to monitor and have an LED illuminate when
an upper or lower limit is exceeded. If you swap the polarity of the op
amp inputs (the - and + inputs) it will operate in reverse - the LED will
be ON when the input voltage is within the window set by the two pots.

Have fun and good luck
72 de NA5N (

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