craiguss Posted March 9, 2012 Share Posted March 9, 2012 Hi,Basically I want to install a comms unit as cheaply as possible, we currently use two way radios, which are good to find where someone is, but thats about all, the quality of sound and interference just isn't what we want, Im in a school and we are on a budget, I was thinking, is there a way of powering a comms unit without the base unit or power unit, could we wire it straight into a mains socket or something?? I know you can now get battery powered belt packs, but they seem to be a lotMore expensive and a power unit would still be cheaper, Please help me, I can't deal with the two way radios anymore!! Thanks!Craig Link to comment Share on other sites More sharing options...
Brian Posted March 9, 2012 Share Posted March 9, 2012 There's nothing particularly clever in a comms PSU. It has two parts... 1) 24 volt DC power supply. Usually with some protection in case of short circuits on the comms line. 2) Audio line termination. Made with a couple of resistors and capacitors; in small systems you *may* get away without these. A small laptop style PSU wired into a plastic project box is eminently feasible. Link to comment Share on other sites More sharing options...
timsabre Posted March 9, 2012 Share Posted March 9, 2012 You do need the audio line termination or the whole thing becomes very loud and unstable, also the call lights often stay on all the time. The termination is simply a 2K2 resistor between the audio line and ground, in parallel with that is a 1uF capacitor and 220 ohm resistor connected in series with each other. Link to comment Share on other sites More sharing options...
paulears Posted March 9, 2012 Share Posted March 9, 2012 As a member of staff - just watch out for any little known rules about building mains powered equipment. Most places have no issues, but I know a couple for schools where they take the lack of a CE mark to be a PAT fail - so prototyping is ok, but putting into service isn't? Keep an eye out for the Canford DC box, they sometimes pop up on ebay and are 12V powered. Link to comment Share on other sites More sharing options...
J Pearce Posted March 9, 2012 Share Posted March 9, 2012 Paul, the easy way round that is to use a separate psu and just build the line termination unit.There are plenty of plug top 24V PSUs available from CPC et al. Link to comment Share on other sites More sharing options...
boatman Posted March 9, 2012 Share Posted March 9, 2012 I used a wall-wart 2A PSU from CPC for mine. The schematic for a single circuit interface can be found here. Link to comment Share on other sites More sharing options...
Brian Posted March 9, 2012 Share Posted March 9, 2012 It's worth mentioning that for best noise performance the output of the 24v supply should not be earthed. You might want to look at some sort of AC grounding to keep RF out of the system though. Link to comment Share on other sites More sharing options...
craiguss Posted March 9, 2012 Author Share Posted March 9, 2012 hi guys,thanks for your such quick reposonses!!im not exactly amazing with electrics, do you have any wiring diagrams or know where I can get some?for protection, could I just put a fuse in or something? that may be a very silly question, but I don't really know what I could wire in to make it protected! =/ so you say not to use the earth from the psu, so do I just not connect it? and earth it some other way? also what is a audio line termination? how would I do that...I'm sorry for being so stupid! aha! Link to comment Share on other sites More sharing options...
paulears Posted March 9, 2012 Share Posted March 9, 2012 No - it means that the pin 1 connection on the power supply is NOT connected to the chassis or from that through to any mains earth - it's not a problem if you use a two pin DC power supply - which only have a + and a - out. Have you seen Boatman's link above - that's quite a useful one, quite clear. If that confuses then I'd not try it, and just buy one ready made. The idea of a ring intercom is similar to the idea of a 100V line loudspeaker system - the devices you stick across the audio line are high impedance, and the audio circuit is held lower by the application of a load resistor/capacitor, so when you add an extra pack, the line impedance only changes a little bit, and you can add more packs. The limit is reached when the power supply circuit is supplying as much as it can. The small resistors & capacitor, which can be seen in Boatman's link across the audio line maintains the line characteristics, even though the number of packs is variable. The gain of the audio system is matched to the incoming audio line - if a dodgy lead somewhere in the system breaks the audio line from the power supply (where the terminator is) the line loses it's low impedance and the gain of the packs goes right up - things feedback, or break out into strange shrieking sounds. Lighting DMX circuits work in a similar way - hence why they always need a termination on the end for things to work properly - the terminator has the lighting version of the audio termination device - in this case a simple 110 Ohm resistor! Timsabre gave the values for the three components making up the audio terminator Link to comment Share on other sites More sharing options...
Recommended Posts
Archived
This topic is now archived and is closed to further replies.