By R. A. E. Milton
After reading the review of the E.V. Bleeper in Vol 2, Iss 1, I realised that
this could be a useful addition to my Nascom, however at £12+ I thought this
was a bit pricey. So I decided to take the cheap way out and design my own,
it turned out to be so succesful that I thought other readers may find it
As I have not got an IVC I’m not sure what sort of signal is available to
trigger the bleeper, but the circuit below is quite simple and could be easily.
adapted to work from any input. The rising edge of a pulse will trigger the
monostable IC1 and this will make the output go high for a period determined
by the setting of VR1. Whilst the output of IC1 (and pin 4 of IC2) is high
then IC2 will oscillate and produce a two tone warble from the speaker, the
tone of the warble can be altered by VR2. The volume is controlled mainly by
the type of speaker used. I found an old telephone insert with a DC
resistance of 80 ohms which works well, but a conventional speaker can be
used. I’ve got my bleeper hanging on bit 2 of Port 0 where it is activated by
sending this bit high, this is simple in machine code or BASIC.
Whilst on the subject of hardware, many moons ago I built the graphics mod
(INMC80 issue 5) and found that it worked very well but gave very small
vertical gaps between continuous characters. On my Nascom 1 this was overcome
by changing IC18 from 74LS123 to an ordinary 74123 and replacing R6 with a 10K ohm
preset, this can then be adjusted so that it just removes the gap without
the characters breaking up. It’s probably a bit naughty but its been working
for a year with no trouble.