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Ah... in this case it was easy...
MegaAdy earlier today asked me for a schematic off-list reminding me of that thread... as I was about to reply I noticed he had posted the question again on-list as well, so I posted the schematic publicly. So no great feat of mental prowess in this instance... and all this from a girl who mislays her car keys a dozen times a week!
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Just looking at that schematic, it may be necicary to note that you will get a voltage drop accross the R1 resistor as well as a voltage drop across the diode. You would want to make sure the voltage getting to the slave pic is sufficient.
As for the large capacitor, you can get 5v, 0.047F (or 47000uF) caps. They are small, quite cheap, and will maintain power for your PIC for a number of seconds.
Last edited by Kamikaze47; - 23rd November 2005 at 19:59.
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Don't worry Melanie, we can fix your car keys problem...
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Surgically implanted obviously! Great idea. Now all I got to do is get all the locks in my life changed to be chip proximity activated.
Kamikaze...
The Voltage drop across R1 will be negligible once C1 has charged since the micropower PICs draw only a few uA. However I do have issue with Caps that size... they'll take forever to charge and the voltage rise-time at the Slave end will be so slow that the PIC will not initialise for the best part of a minute! Just work out the CR time constant!
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You disappoint me MelanieOriginally Posted by Melanie
After all those superb code examples, I thought you had some magical formula for finding previously written information
Good to know you suffer from the same problems as the rest of us.
I have lost count of the number of times I have put down a screwdriver and two minutes later when I go to pick it up it isnt there!
Nice solution to the problem BTW. My initial thoughts were for a choke at each end and use a carrier to modulate a tone onto the power rail. Much higher component count and cost.
Regards
Keith
www.diyha.co.uk
www.kat5.tv
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Thank you for schematic Melanie ! You were right, I posted this question some months ago. You didn't understand my setup here. So, once again,
I have two devices ( not pics ). A master device, and a slave device that get power from the slave (5v, 3-400 mA ). Now, I want to put a PIC in the slave device which should send to the pic I put in the master device a few bytes ( 1200 baud ). The problem is the power consumption of the slave ( 4-500mA). And I can not use too big caps like ... 4700u@25v. I can not test the schematic right now, but I got the ideea. But do you thing it will work in the context I explained you ? The power on that slave device ( and the transmitting pic also ) should be kept as close as it can to 5V. Thank you .
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If your Slave device is power hungry, then the only way is to modulate a signal onto the Power Supply lines as suggested by Keith. The schematic I've posted would not be appropriate in that instance.
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hm ... can you please be more specific ?
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I dont think you can even use the modulation technique in this instance because its the slave device he wants sending the data (i.e. the device that doesnt have a power supply).
You can use the mondulation technique if you were transmitting data from the master to the slave, but not slave to master.
If you already have 2 wires between the 2 devices, whats so hard about running a 3rd wire for data?
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You got the point ! That's why I thought it might not work ! Although, someone did it, I don't know exactly how... Forget the 3rd wire...I just can not use more than 2 wires
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I beg to differ.
The data would be used to turn on and off a carrier which would become an AC signal. This can be superimposed on top of a DC signal quite easily.
It doesnt matter which direction power or data are heading. They simply use the same piece of wire as their connection.
A phantom powered electret microphone effectively does what is requested here. Power is fed from the mixer (MASTER) whilst the audio (or Data) is fed along the exact same wires by the Microphone (SLAVE).
See attached diagram for how data and power would be connected. I havent draw the modulator and demodulator as I dont have a suitable design to hand but generating the signal could be as easy as running the PWM on the SLAVE PIC ad using a transistor driven the the TX data pin to modulate it. At the MASTER end you would need a circuit to detect the carrier and provide a logic level to drive the RX data pin on the Master Pic.
The inductors allow DC power to pass but block the AC carrier so that PSU compoments dont attenuate the carrier.
The capacitors allow the AC carrier to pass but block the DC supply voltage.
Keith
www.diyha.co.uk
www.kat5.tv
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