LandyManLuke Posted September 11, 2006 Share Posted September 11, 2006 Evening, I need help designing a circuit, I have some resistor-bridge pressure sensors, and i need to get an output that i can feed into a 0-5vDC ADC. The sensors give about 100mV across the bridge at full deflection, ie 50mV either side of 0v. Just to complicate things a little more, i want to multiplex three senors into one ADC, selecting each sensor by pulling an input down to 0v. Any help!? pweeese Thanks Luke Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted September 11, 2006 Share Posted September 11, 2006 If you know what an op-amp is you're laughing, if not then you should probably ask a grown-up Quote Link to comment Share on other sites More sharing options...
LandyManLuke Posted September 11, 2006 Author Share Posted September 11, 2006 Hi Fridge, I was hoping you'd reply. Yes i know what an op-amp is, i know what it does, but ask me to design a circuit such as i need above, and i'm stuck. That's the trouble with doing a degree in Systems & Control, rather than Electronics. Can you help at all? Cheers Luke Quote Link to comment Share on other sites More sharing options...
rtbarton Posted September 11, 2006 Share Posted September 11, 2006 Don't believe analogue electricty exists, but there are (used to be) chips called CMOS switches which will switch analogue signals (whatever they are) I suspect you can get multiplexors to switch one of many inputs to a single output. PS Luke, my BT BB is up to 250 kbs after the thunderstorm tonight! Quote Link to comment Share on other sites More sharing options...
LandyManLuke Posted September 11, 2006 Author Share Posted September 11, 2006 fan-dabby-dozy! now i just need to find them, and get the signal to scale from 0v, rather than either side of it!! Yuh, analogue sucks, digital is much nicer. shame i couldn't find direct pressure to digital sensors, that would have been nice. - my BB is still at 130 kbps, perhaps the lightening was more over your bit of sheffield!! Luke Quote Link to comment Share on other sites More sharing options...
simonr Posted September 11, 2006 Share Posted September 11, 2006 Wahat you want is a 4016 - quad bilateral switch chip. This contains four analogue switches on a chip - ideal for this type of application. Depending on the degree of accuracy you want, you could just use a simple one or two transistor class A amp. Si Quote Link to comment Share on other sites More sharing options...
LandyManLuke Posted September 11, 2006 Author Share Posted September 11, 2006 Hi Si, I'll look up the 4016. The sensor's resolution is about 0.85 mV/PSI, I'm using the sensors for my air compressor set up, so max pressure is about 120 PSI, so ~100mV is maximum output. The ADC on the USB experiment board i'm using has an input range of 0-5vDC, although it does have an onboard op-amp i can tune if needs be. the bit i don't know where to go on is converting the +ve and -ve form of the sensor output into a '0 to something' form, for the ADC. Luke Quote Link to comment Share on other sites More sharing options...
rtbarton Posted September 11, 2006 Share Posted September 11, 2006 4016 that rings a bell! http://en.wikipedia.org/wiki/Operational_amplifier Quote Link to comment Share on other sites More sharing options...
LandyManLuke Posted September 11, 2006 Author Share Posted September 11, 2006 What's even nicer is maplin have them in stock at the shop in town, looks like progress! Luke Quote Link to comment Share on other sites More sharing options...
rtbarton Posted September 11, 2006 Share Posted September 11, 2006 If the sensor is a resitor bridge you could wire it as a potentiometer instead, taking the signal from junction of two resistors and ground. You'll probably get less voltage swing, but you're having to build an amplifier anyway. And I'll blag the circuit off you when you've got it working! Quote Link to comment Share on other sites More sharing options...
LandyManLuke Posted September 11, 2006 Author Share Posted September 11, 2006 Yup, i've fiddled with that this evening, swing is pretty pitiful - about half (as expected), and i wonder if there are any other effects in terms of accuracy. I'd like to keep it as a bridge, if i can, though i realise it's simpler to do it the other way. Luke Edit: Having read a bit more about op-amps, it looks like that's the way to go, using both the inverting and non-inverting inputs to amplify the difference across the bridge, the question is, how?! Edit: Yey, Wikipedia has helped me along a bit, now i need to find an Op-Amp package that will do the job. I need it to have 0Vdc and +9Vdc suppy rails, preferably, and low saturation on the output, as i'll need it to work down near 0v. When R1 = R2 and Rf = Rg Can anyone suggest a suitable package, and what sort of range the resistors need to be in? ie in the Kohms? lower? higher? Cheers Luke Quote Link to comment Share on other sites More sharing options...
LandyManLuke Posted September 12, 2006 Author Share Posted September 12, 2006 Whilst we're at it, does anyone know a chip that'll convert two bit binary into 4 outputs? i.e. put 00 on the inputs and the 1st output goes high, put 11 on the inputs and the third output goes high, etc etc. that way i can cut down the number of control lines from the PC from 3 to 2. Crikey i should get some sleep Edit: 4051 - 8 way analogue multiplexer/demultiplexer, really tired now. Luke Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted September 12, 2006 Share Posted September 12, 2006 Well TL082 or the classic 741 would be first choice, or you could amplify the signals before the analogue switch to minimise any loss across it using an LM339 quad op-amp. If you need this prototyping / soldering together / making a few of then drop me a PM. Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted September 12, 2006 Share Posted September 12, 2006 For decoding you could use a 4515 1-of-16 decoder, a small waste of it's talents but much easier than doing it with discrete logic. Mind you, the other solution to all this is to use an HC908 or a PIC and just do everything in one chip. It does involve programming though Quote Link to comment Share on other sites More sharing options...
rtbarton Posted September 12, 2006 Share Posted September 12, 2006 Whilst we're at it, does anyone know a chip that'll convert two bit binary into 4 outputs? i.e. put 00 on the inputs and the 1st output goes high, put 11 on the inputs and the third output goes high, etc etc. that way i can cut down the number of control lines from the PC from 3 to 2. That's a standard decocoder although you'll probably not get a 2 bit input. I'm sure all the various functions are listed in the Maplin catalogue, typically I binned all my data books years ago. http://en.wikipedia.org/wiki/List_of_7400_...grated_circuits Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted September 12, 2006 Share Posted September 12, 2006 Binned data books? Shame on you! Quote Link to comment Share on other sites More sharing options...
rtbarton Posted September 12, 2006 Share Posted September 12, 2006 Binned data books? Shame on you! Ebayed actually IIRC, still didn't get a fraction of the cost back Quote Link to comment Share on other sites More sharing options...
LandyManLuke Posted September 12, 2006 Author Share Posted September 12, 2006 Morning (afternoon?) Yup, my intention is amplify before the switch. I've found the 4051 which apparently is a 8 channel analogue MUX/DEMUX, i can just hold the MSB low and use the first 4. I'll look up a datasheet for the LM339, looks like that'll do the job. Fridge, only the one needed, half of the rest of the circuit is already on veroboard and working (kinda), thanks for the offer though. I could go down the pic route, i'm fairly fluent in VB and C, but i think i'll stick with simple logic for now. general project background here Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted September 12, 2006 Share Posted September 12, 2006 That looks, erm, complicated... what are you trying to achieve exactly? Have you ever heard of Hyphose, they will sell you proper air solenoids, plumbing, etc. and probably have sensors with a nicer output. Quote Link to comment Share on other sites More sharing options...
LandyManLuke Posted September 12, 2006 Author Share Posted September 12, 2006 What am i trying to achieve? not much really! the fun is in getting it to work. I've done quite a lot of research into air solenoids, and very few of them are truely bi-directional, they are also rather expensive. I've already got three of these sensors, and at a tenner each, i'm reluctant to throw them away. I'll have a look at Hyphose. Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted September 12, 2006 Share Posted September 12, 2006 OK, put it another way - what does this contraption actually DO? Quote Link to comment Share on other sites More sharing options...
LandyManLuke Posted September 12, 2006 Author Share Posted September 12, 2006 basically, I'm using a 50L tank and 1.5 Hp compressor to fill the 300 litre tank when i'm not in the garage, the trouble is the standard pressure switch would knacker the compressor as it's duty cycle would be exceeded filling such a big tank. The PC can cycle the pump over time, keeping it from over heating, whilst filling the tank, also, in the future, i'll add a freezer compressor to top up the tank over longer periods of time, say overnight. When i'm in the garage, the compressor is deafening, having 350 litres of air means i can use a fair amount of toolage before seeing a significant pressure drop, also, a freezer pump could replenish the tank quietly. pressure sensing means i can calculate how long i've got until pressure drops too low, i can also run the pumps to meet demand. the valving means i have to option of using the small tank whilst filling the big tank with the freezer motor etc etc. Luke Quote Link to comment Share on other sites More sharing options...
Turbocharger Posted September 12, 2006 Share Posted September 12, 2006 Filling overnight? I bet the neighbours will love you Why not just add an overtemp switch to the compressor motor, switching it off when it gets vaguely warm? Simpler = better. Quote Link to comment Share on other sites More sharing options...
LandyManLuke Posted September 12, 2006 Author Share Posted September 12, 2006 overnight filling would be done with the freezer compressor, daytime filling with main compressor, easy with a bit of programming. you can't add a switch to the motor that simply, as it wouldn't control the dump valve for compressor startup. I've got a seperate solenoid to dump the air on compressor start up. Quote Link to comment Share on other sites More sharing options...
rtbarton Posted September 12, 2006 Share Posted September 12, 2006 Filling overnight? I bet the neighbours will love you Why not just add an overtemp switch to the compressor motor, switching it off when it gets vaguely warm? Simpler = better. The freezer motor(s) will be used to fill the tank overnight. I do this on Economy 7 with a bank of 5 assorted Fridgefreezer motors. The problem with just switching the motor off is that you also need a dump valve between the compressor and tank so that the motor doesn't restart under pressure. edit - simultaneous post Luke !! Quote Link to comment Share on other sites More sharing options...
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