|Digital talk box with accurate timer.|
A Digital Talk Box with sequence timer.
The concept of this lump of electronics is not new. A friend built one several years back and
this would put out a short audio transmission periodically with the intervals between transmissions
being set by a variable resistor that controlled the operating frequency of a 555 timer.
My Christmas project was to put together a design that would fully satisfy all the requirements of
the three scenarios.
1) It must have an accurate but adjustable sequence timer.
2) It must be able to hold at least 1 minute of speech
3) It must have outputs to suit all radios, including handsets and those without VOX capability.
Catalogues were perused an eventually the lowest cost speech chip capable of holding 1 minute of speech was one that held 64 seconds and was sold for £9.10 by RS Components.
Stock number RS 252-1284
The timing chip was a bit more difficult to identify. With my disdain for software totally ruling out suggestions of using a PIC processor I ended up using a Euroquartz Programmable quartz Oscillator type SPG 8650B. Cost about £7.50 from Euroquartz on 01460 76477. (Nice bloke there is Steve Jarvis G4TIA) This chip has a 6 bit wide address so using bit switches one can select any output frequency between once every 20 minutes down a frequency of 100 Khz.. More importantly times of 1,2,3,4,5,6,10,12,30,40,50,60 seconds and 1,2,5,10 & 20 minutes could be selected.
This range should satisfy pretty well most applications!
Circuit description :-
The interface to the radio is via a pair of 8 pin DIN connectors, one to connect to the radio via a patch lead and the other to a microphone if required. These are wired to suit a Raynet format known as CAIRO. In essence the audio output is totally isolated using a small transformer so that it can be connected to any rig via the microphone input - even handsets where the microphone is connected in series with the PTT switch. Other connections include a 12 volt supply and some connect input to output. The audio output from the chip is a bridge output capable of delivering 50mW into a 16 ohm loudspeaker. Interestingly, when audio is being output the two connections pop up from zero volts to about 2 volts +/- the modulation, then back to zero at end of message. This DC voltage was used to drive the PTT transistor that controls the coils of two 2 pole relays.
When the relays are de-energised one pair of poles connect the external microphone directly to the radio. Another set connect the PTT line of the radio to the PTT of the microphone. When these relays are energised the output from the microphone transformer is connected to the radio and the radio's PTT line is connected to negative to cause the radio to transmit. The fourth set of relay contacts were going to be spare but a learning curve on the requirements of the reset line of the speech chip meant that these had to be used to generate a positive pulse to "reset the stack" as those computer specialists would say.
Two transistors make a bi-stable. This is used to enable or disable the crystal based sequence timer. I could have used a chip but I have and am happy with transistors and I would have needed to use transistors to interface a chip anyway.
The reason for the bi-stable is that as well as having ON and OFF AUTORUN buttons it was also necessary to arrange things so that should the PTT line of the rig be taken low externally that the sequence timer must also stop. The sequence can only be restarted by manual operation of the AUTO START button. This is necessary for contests when in that short listening gap between transmissions someone comes back to you. Grab the mic and talk. When you have completed your QSO re-start the timer by pressing the button.
Manual START and STOP buttons are also provided for the speech chip for single shot operation or for recording your OGM (out going message). An electret mic is incorporated onto the PCB. Lastly two toggle switches are used to select PLAY or RECORD and TEST mode where you can play your OGM without putting the radio in transmit.
All buttons needed to be more user friendly - so, most of us have an old computer keyboard that can be canibalised and these keys are ideal. Light weight, designed for the fingers and intended for lots of operations!
My last requirement was that the whole lot must be on one printed circuit board that fits into a snug little die cast aluminium box. Even the 8 pin DIN connectors. So no wire rats nest like Mk1. i.e put the bits on the PCB, solder it together, switch ON and go!
G4ODM. May 2003
Board layout viewed from component side.
Return to home page