looking for a timer circuit

google on: "sample and hold" schematic

That should bring up a fair number of results.

HTH

I have (and still am) looking through pages of google results. So far, nothing that looks usable to me :^). I'm trying to find a circuit that will stabilize an air/fuel ratio gauge display (a series of LEDs). The gauge takes its input from an O2 sensor in the exhaust header (single wire type). The o2 sensor varies it's output from 0VDC to 1VDC based on it's reading. The problem with the gauge is it samples too frequently and the display is almost useless. If I could build something that takes the output from the o2 sensor and 'level' it (only take 1 reading per second and output that reading to the gauge until the next reading), I think it would stabilize the display and make it more usable.

Thanks,

Wayne

On Tue, 11 Mar 2008 07:37:17 -0700 (PDT), Wayne wrote: . . .

--
View in Courier:


.[O2 SENSOR]---[ADC]---[LATCH]---[DAC]---[DISPLAY]
.                |        |
.[1s CLOCK]------+--------+

You may instead want a way to average the result. Otherwise, you end up with the 'random' result being displayed for 1 sec, followed by another random result, etc.

You can do this with a low pass filter, perhaps. The simplest is a resistor from input to output, with a capacitor from output to ground. The values to use are dependent on how much you want to smooth the output. If you want the output to average out at 1Hz or less, then you need components so that

R * C = 0.16 = (1/(2*PI))

So, if you use a 1uF capacitor, you would need about 160k of resistance.

So, here is the circuit:

160k sensor ---- RRR ---o---- output | --- 1uF --- | GND

Regards, Bob Monsen

=46rom this it sounds like the problem is that your signal is bouncing around a lot. Many samples of a steady signal would not be an issue. So rather than just taking snapshot samples and displaying them, you may want to do some averaging on the input value.

Me, I'd be inclined to do the whole thing with an 8-pin micro e.g. AVR. How much resolution does your display have?

Mike

Yes, this is definitely the first thing to try. I got caught up in the whole input-hold-average-output thing.

Mike

Well, using a simple RC type padding will slow down the movement of the signal for you how ever, if this sensor is connected to another device in the car like your car's ignition computer, you may want to isolate the signal before altering it to suit your needs. Passing the signal into the + input of an op-amp and on the output of the op-amp you apply the padding RC filtering.

"

How about a kit from Jaycar Electronics

Kit number KC5195

should take you to the page

I think it does exactly what you want and there are loads more toys to buy to make ordering from Australia worth while.

I'll also add my vote for the filter approach as a first step.

But if you ultimately decide you do need a sample/hold, it can be done in a fairly straightforward manner without going digital. You have a buffer amp that charges a capacitor through a switch (4016, etc). The capacitor voltage is monitored by another buffer that feeds your display circuit. A timer closes the switch for a brief interval, once per second, to charge up the cap to the present input voltage.

The input impedance of the op-amp output buffer will be very high, so it won't draw down the capacitor voltage between samples. In general you need a low input bias current op-amp for this, but I'll bet a TL082 or LF351 will be OK. You will need to experiment with the cap if dielectric absorption or leakage gives you problems, but I suspect a big mylar will be fine since this is not really a critical issue for your application.

The timer is totally non-critical. I prefer simple CMOS gate timers, but you could probably use the popular (though I could never understand why) 555 series here if you don't mind all the extra parts and the supply current spikes.

Best regards,

Bob Masta DAQARTA v3.50 Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, FREE Signal Generator Science with your sound card!

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Bob,

I tried your suggestion today, and it did help some. You can definitely see a difference in the display, especially at idle. Thanks.