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Need a Time Sharing Circuit

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Full Member level 2
Aug 13, 2001
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I have an optical sensing system that uses three LEDs (LED1, LED2 & LED3) as transmitters. The lights from these LEDs are detected by three Photodiodes (PD1, PD2 & PD3). The signals from the PDs are passed through an Analog Circuit that is common to all. Then the analog output is passed through three different digital circuits Dig1, Dig2 & Dig3 for indication. At present, I'm using only one sensor at a time (LED1->PD1->Analog->Dig1) by manually chnaging the connections. I would like to have a system that will keep LED1->PD1->Analog->Dig1 ON for 30 secs, then turn it OFF, turn LED2->PD2->Analog->Dig2 ON for 30 secs, then turn it OFF, turn LED3->PD3->Analog->Dig3 ON for 30 secs and then turn the whole system OFF for 10 mins. 10 mins later it will do the same cycle again. Is there any way to do that? I need to do this to save the battery and also to use only one Analog Circuit.

What about a pic micro and some fet switches?
Should do that quite nicely.

Could you plz explain in details!

This is not hard to do. Use a current amplifier after each PD and then switch them through either an analog gate or using discrete Mosfets. Then use another analog gate on the output of the analog circuit to connect it to digital 1,2 or 3. In terms of the timing circuit, use a digital counter with about a 5Hz clock. Use multiple input AND gates to pick the right counter outupts for each timing event.

Why don't use a CPLD like Xilinx XC9536 or XC9572 its flexible and you can do this.

Second optiion (if speed is low) is using any microcontroler.

I think the micro solution would be the easiest, most flexible and use the lowest current. You can buy photo-diodes with built in drivers. They contain all the circuitry to give a logic level on detection.
Power these devices from a common rail, but take their ground connection to an input on a microcontroller (microchip pics are perfect for this). Take the output to another pic input. To activate each sensor, pull the port (ground connection) low and read the input. Do this for each sensor and therefore you can control the devices seperately and keep the current low.
Alternatively if you need an varying analogue output from the photo diode, you can, if the output levels are enough drive the photodiode output into one of the a-d inputs on say a PIC16f877 and then do what you want with the signal in the processor.
Microchip do a ICD development system, ICD1 uses the PIC16f877 (this is good ut has pages which can confuse a novice programmer).
Use the new ICD2.

Hope this helps


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