Wow, a long time with no posts – sorry about that!

I have been busy the past week, I wanted to take Thursday and Friday off, so I had to cram a weeks worth of work into the first three days.

I’ve also been racking my brain, trying to figure out how I can integrate a matrix wired LED array into my sensor project. I think I’ve come up with a solution, partly hardware, partly software. Originally I had wanted to have all the LEDs on, and turn them off to do a measurement. This created a lot of problems, mostly due to the current demands. Well, I think I can achieve the same goals, without having all the LEDs lit simultaneously, which will reduce the current demands and should simply the circuit a little.

There is the PCB layout for my latest design… I think I’ll make one to test out, and if it doesn’t work for my sensor array, I can use it as a mini marquee. Sixteen vertical columns, anode connected, each driven by a 74HC595 serial shift register. Seven horizontal rows, cathode connected, each driven directly by the PIC’s analog input / digital output pins. To keep current demands low, I will only illuminate one column at a time. I think that still exceeds the design recommendations of the 595, but heck, its all I can think of at the moment.

My current ‘test’ setup with a 3×3 led matrix

prototype driver for the cathode rows … will never see the breadboard however, since it won’t pass-through an input signal to the PIC.

prototype driver for the anode columns… I’ll breadboard this out soon as I get some p-chan fets in.

More than a few days have passed since my last posting. What have I been working on? Well, the same stuff.

I’d been getting my a** kicked by a logic gate thingy for two days, and turns out, all I had was two wires transposed – ahh well. Here is a re-post of a thread I started on linear1.org:

As part of my continuing effort to develop LED sensor technology, I’ve been trying to tackle the problem of expansion… My original brute-force method was an army of PIC microcontrollers, discretely controlling individual LEDs. While this method worked and was rather simple from a hardware stand-point, there were several drawbacks. The most ADC ports I was able to use was ten… there are microcontrollers with more than this, however they have certain drawbacks that make them unsuitable for my application. Knowing the limit of 10 leds per slave, to build an array of a useful size, say 3×10, would require three slave microcontrollers. Although not enormously expensive, each mcu costs at minimum $3, which adds up quickly.

The method I am currently exploring involves linear logic chips, specificly the 74HCT595N 8 bit serial load latching shift register. Here is the idea, and I’ll throw up a schematic shortly. The cost of a 595 for hobby purposes is roughly 75 cents. Even a modest quanity of 20-25 shifts that price down below 50 cents a piece. Far better than the cost of individual microcontrollers.

The 595 is used to source current, driving the anode axis of the array (columns). I fill the register with 1’s, which sets each output as a current source. While filling with 1’s, I clear the bit for the column which contains the LED I want to sample. The 595 holds this state indeffinately, until it is changed by the microcontroller. The microcontroller, which is directly interfacing the cathode axis of the array (rows), then samples each LED in that column by applying a reverse bias charge and then switching to a high-z input for the A2D reading for each row. This is done one bit at a time (each bit equals a row), so neighboring rows are left “on” at are lit, allowing a good amount of light to be reflected into the led currently being read.

This new arrangement allows a single microcontroller to handle a matrix of ‘unlimited’ columns by ten rows (each row needs to be connected directly to the microcontrollers unique tri-state IO port). A 160 pixel array of 16 columns by 10 rows needs only one microcontroller and two shift registers. Of course, the actual size of the array will be determined by how fast the PIC can scan through the columns, since one column is always ‘dark’ to allow for the reverse bias and read operations. Currently my microcontroller can scan my 3×3 array at a rate of 27kHz, which gives me a WIDE margin, since anything over 70 hz is practically impreceivable.

There are some flaws I need to sort out, mainly power related. The 595 can source 35mA and sink 75mA. 35mA is fine for a 3×3 array, but it won’t cut it for anything much larger. I need to find a way to increase the current handling abilities of each port, without a huge component count or complexity. If I build an array with 10 rows, each column needs to source 200 mA of current. But it also needs to retain the ability to sink a small amount of current. I figure I can solve this with a pair of transistors, one small npn for sinking, one larger pnp for sourcing.

Secondly, my microcontroller can source 25mA of current which is more than enough to reverse bias an led quickly, but it can only sink 35mA of current. So, when sinking a row that may be 16 or more columns long (at 20mA per led), some serious power handling capacity comes into play here. A simple TO92 mosfet can sink an amp of current without breaking a sweat, but I also need the ability to source current and the ability to go “high-z” to allow the microcontrollers internal circuitry to do the A2D sampling.

I think I am close to lickin this issue. More to come.

Well, I’m having a some code-writers-block on applying data from the array to anything usefull… so I’m looking at ways to get more data from fewer and cheaper microcontrollers.

The cornerstone of led sensing is tristate logic. The PIC microcontroller can set each of its pins to high, low or neither (high-z). This is what allows me to forward bias, reverse bias and then sample the led. However, doing all of this requires a large number of pins on the PIC. Two pins are required for each LED. That’s why I did all this work on networking pics on a shared bus… but the cost of pics is pretty extreme, so I’m wondering about alternate ways of accomplishing the same.

Enter low-cost analog and logic interface ICs. There is a device called the 74HC573 octal tristate latching buffer. Basicly its a chip with 8 outputs, which can be high, low or high-z — same as the PIC. I really don’t care about the number of outputs, I plan to use a pair of them in an all or nothing configuration… tie all the data input lines together with a single PIC pin. I also probably won’t need the latching ability, but the 573 was cheaper than the 244 which is an octal driver without latching. So, the pic can control 16 led anodes with 1 pin – thats some awesome savings right there! 🙂 Next comes the cathode side, which is what the PIC reads with the ADC. For driving the cathodes, I plan to use the same configuration as the anodes, with the exception that I’ll also use the output enable line on the 573, to put the outputs into high-z just before sampling. All this leaves is the actual sampling. I’ll do this with a 16 to 1 analog multiplexer. The mux has a pretty high R_DS(ON), something like 180 Ω … which is actually pretty close to the 150 Ω I’m using now. I figure I can put my resistor on the cathode side, after the mux connection, so when the cathode side driver is in high-z mode, my current limiting resistor disappears. In order to keep the array bright and eliminate any humans from seeing flickering, I’ll flash the entire row before sampling each LED. This should also give me the best conversion resolution on my leds, since the sampling time needs to be so high, I wouldn’t be able to read them all before their values changed.

By my calculations, scribbling in notepad, I figure I’ll need nine lines from the pic to control 32 LED lines (anodes and cathodes for 16 leds). Which is great, since I can use the lower-priced 18pin PICs… and it also sets the default row size to 16, up from the current 10. Of course, there’s no reason I’d need to do the full 16, but its good to have options! For the cost of one more pin from the pic (still easily within the 18 pin range), I could add another 16:1 mux, and lay 32 leds on a single pic! With the pic able to source and sink at least 25mA; it should be able to drive a ton of 74HC573s, which according to the datasheet need 1μA of input current per data line. Muhahahah! I’m not sure how many LEDs each PIC can handle and still have enough time to get a few complete collections inbetween servicing i2c interrupt requests with a 400 or 1000 khz bus clock.