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A/F Gauge
I wanted a A/F Gauge to monitor the mixture in my car, and didn't want to spend $30+ for something that just measures voltage from the O2 sensor wire in the car. I'm sure that Cyberdyne makes their A/F gauge the same way this is done, but this one costs about $4.

The parts list is:
National Semiconductor LM3914N LED Bar Graph display driver chip ($3.75 Digi-key)
National Semiconductor LM340T-5.0 Voltage regulator ($1.19 Digi-key or Radio Shack #276-1770 $1.19)
10-segment red LED bargraph (Radio Shack #276-081 $2.99)
Miscellaneous resistors (~$1)

Optional items that may be of interest:
Multi-Color 10-Segment Bargraph [GGGYYYYRRR] ($3.47 Newark Electronics, Part #91F4665, 800-4-NEWARK)
10-Segment Bargraph with Driver [Red only] ($8.53 Farnell Components, Part #324-061, 800-718-1997)
Right now you are asking yourself, how did he get $4? I see it costing $8.93! Well it so happens that if you go on to National Semiconductor's homepage, you can request a free sample of their products. Do a search for LM3914N and LM340T-5.0, and you'll see where you can order these items for free at the bottom of the page. They reserve the right to refuse your request, so be sure that you put something applicable to electronics in the job title section (something with "engineer" would be adviseable). Note (9/3/97): I have heard from people that have tried contacting NS that they have discontinued the 'free sample' policy. So you may have no choice but to buy them [Radio Shack does NOT sell the driver chip].
 
See circuit diagram.
 
The first resistor, R1, adjusts the range of voltages that the IC should expect. In our case that is about 1.0 volt (actually I ran my car hard and had a max reading of 0.981 volt). With the resitances shown, my reference voltage is 0.97 volt. This means that at a reading of 0.97 V, the 10th LED with illuminate. The second resistor, R2, determines the brightness of the the LEDs. This can be adjusted to suit user preference.

LED numbering:

The first LED to light (LED1) will be the LED connected to pin #1, then LED2 (pin #18), then LED3 (pin #17), and so on til LED10 (pin #10). When working properly, a lean mixture is LED 1, and a rich mixture is LED 10.

User Preference:

The way it is currently set up, only 1 LED will be lit at any time (called 'dot' mode). The higher the voltage reading from the O2 sensor, the higher numbered LED will light (but only at a time). If you wish to have the display be a 'bar' instead of a 'dot', just connect pin #9 to pin #3. This will light the current LED and all others below it.

Connections, Installation, and Readings

I was able to fit the entire circuit on a 1.75"x1.75" piece of circuit board (#276-148). I bought a piece of disk drive extension cable (#278-767), and cut it in two and used one piece on the circuit board and the other I wired into the car. Then when I was ready all I had to do was "snap" and it was connnected. This will make the unit easy to remove if there is a problem.
I installed my unit under my AM/FM radio in the bottom DIN slot which was empty. Made a rectangular piece of platic to fit, then using a dremel made a 1"x0.25" slit to fit the bargraph. Mounted the circuit board behind the platic with some aluminum spacers (#64-3024) so the bargraph would come through.
For connecting the 12v power supply input, I used the "gauges" fuse on the drivers side kickpanel fusebox. The ground can be found anywhere and the O2 sensor input is from the connector on the passengers side kick panel. See Finding the O2 Sensor Wire Page. Use a male spade connector (#64-3038) that's 1/4" wide to tap into the connector (assuming you don't want to cut the wire).
These are the readings I get, and are pretty sure that they are correct:
Idle (out of gear): rapidly fluctuates up and down scale
Decelleration (foot off gas but still in gear): lean mixture (almost no LEDs illuminated)
Normal Acceleration: rapidly fluctuates up and down
Hard Acceleration: rich (stays in upper portion of LED count)
When the car is first started, there is almost no voltage from the O2 sensor. After about 20 seconds you should see signs of life from your gauge.

Good Luck!


 

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Copyright 1999, Brad Bauer