I've been thinking about making a simple O2 meter for my car using a LM3914. I've read the datasheet and appnotes and I think grok enough of it. Here's what I'm looking to do. The 02 sensor has a range of 0-1V. I'm happy to use the schematic in the block diagram for a 1.25v scale. Instead of using 10 leds, I was thinking something a bit different. I'd like to use it to drive one RGB led. Personally, I think an 02 meter is of little value normally. But it's good to have some idea of what's going on. I was also thinking that our brains work really well recognizing patterns, so that over time, I might be able to pick up when something changes. A substantially different pattern might stand out to my subconscious. I'd like to have each pin about supply roughly 5.6ma in bargraph mode like so. Pins : LED 1 : B 2 : R 3 : R 4 : R 5 : G 6 : G 7 : G 8 : B 9 : B 10 : (Undecided, maybe light another led) The reason for hooking pin 1 to Blue is that normally, the sensor shouldn't be in the under 250mv range and in normal operation, blue will never be lit by itself which let's it act as a low level warning. So in normal operation, the led should go from red to orange to white, (I may change the ordering but should give you the idea) So it looks like I just have to figure out the resistor for pin 7. Am I thinking correctly that a 2200 ohm resistor would supply 5.6ma to each output? The other thought is, do I have to worry about having multiple outputs connected? Any thoughts or suggestions would be appreciated.
It's very easy to get exactly 1.00V FSD, just put a 1k pot across the 1.25V reference (along with a 1k2 resistor in series to set the led current to 5.7mA) and tap off 1V. I'm not sure how the built-in CCS will cope with a few pins running 1 led, you may need to put your own current limiter or CSS on the 3 led anodes.
