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G8MNY  > TECH     09.03.24 10:02z 45 Lines 1823 Bytes #-2030 (0) @ WW
BID : 2883_GB7CIP
Read: GUEST
Subj: Mono Stable Revisited
Path: SR1BSZ<IW0QNL<VE2PKT<I0OJJ<GB7CIP
Sent: 240309/0955Z @:GB7CIP.#32.GBR.EURO #:2883 [Caterham Surrey GBR] $:2883_GB
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                      (Updated Sep 18)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

I needed a timer to timeout a 0V ON signal, on a Tx after a few minutes for RDS
Trafic flag. I had done it before with a 555 timer & input & output
conditioning inverters, all far too complex really. So I had a re-think on the
basic 2 transistor circuit, & came up with this simple junk box design.

A simular circuit can be used on PTT line of most Tx to limit key up time!

+5-20V >ÄÄÂÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄ¿
         10K   ³              33K     1K
          ³   2M2              ³      _³_
          ³    ³R              ³  LED=\_/      Time  
          ³    ³          BC337ÃÄ¿     ÃÄÄÄÄÄÄ>Limited
Low       ³ +  ³           T1³/  ³   ³/BC337   Low Output
Input >ÄÄÄÁÄ´ÃÄÁÄ10KÄÂÄÄ´>ÃÄÄ´   ÀÄÄÄ´ T2
          C 100u    ===  *   ³\e     ³\e 
                  u1 ³         ³       ³
0V >ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄ>

With no input, T1 is on & T2 held off. The 33K must be low enough to get 0V out
of T2 with required load, & the 2M2 must be low enough for T1 to turn off T2.
Higher gain transistor like the BC109c (Beta = 900) can be used for T1 with R
of 10M for longer times, if the cap C is realy low leakage.  

With the input low, a negative voltage equal to the power rail, is appled to
T1's base & diode, holding it off for around 200 seconds. The CR time constant
of 2M2 with the 100uF cap sets the time, but a leaky C will only shorten the
time slighty in this circuit.

T1s Base 10K & u1 give basic false pulse/RF imunity.

On 5V the diode (* 1N4148) can be omitted as it is not needed to protect T1's
base from going too negative & damaging T1.



Why Don't U send an interesting bul?

73 De John, G8MNY @ GB7CIP


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