Wireless home router with analogue utilisation meter

Step 3 Op-amp circuit

There are two stages between the LED on the main router board and the rev counter.

1) Isolate the op-amp circuit from the router board. This is done with a 'buffer' in the form of a 74HC04. An invertor that has gates which will not draw any current from the router and will output a signal based on the inverse of its input. This guy comes with 6 gates, so if you want to get the same output signal as the input signal you tie to gates back to back.

Missing Link) I had an intermediatary stage that was designed to smooth the square wave signal driving the LED to a nice analogue rising/falling charge to the rev counter, however the mechanics of the counter provided the smooth necessary. So, in some of the diagrams you'll see and RC Low pass filter.

2)The Op-amp. I choose a very old chip, the LM 741, which worked but came with a lot of limitations that drastically affected the design. Note how the rev counter never goes to zero, and the range seems to hover around the center of the dial. Limitations of the op-amp. Leason learned and over the coming weeks I'm going to improve this circuit to have a wider range output.

Comments

[he3r0]

This is awesome, I really love analog gauges like that in computer equipment. I will definitely try to do this since i have a couple of routers that I do not use. Also I have a question, How difficult would be to wire tach to show processor speed?

[Mourtegoul]

Hiya. Is it really necessary to get into OpAmps at all here? I don't really know much about the tachometer, but I'm assuming it displays proportional to the applied voltage from 0-12v or 0-14v from the descriptions. Why not use the square wave output that's driving the activity LED to drive the base of a fairly hefty BJT between one pole of the tach' and ground, and tie the other end of the tach to 12volts? Is the tach' inductive?

[radiorental]

Very true, there is any number of solutions to the problem. Some suggested a microcontroller. The op-amp works, and I will look at a simple transistor. The thing to think about is smoothing, with a flat out sqr wave driving the meter you will notice every little 'blip', I wanted some form of cumulative gain. Even with the op-amp I have achieved the desired output, it still swings a lot. Thanks for your feedback

[thrudd]

Good point about the router power needs.

Simplest solution would be to take your +12v output and split it with a ladder bridge. Energy wasteful but easy to build and pretty much rock solid.
Next is just feeding the +12v into a +5v regulator circuit (chose your flavour).
The last is just salvaging a small enough switching supply with the needed voltage outputs. (ah salvage) =D

[maestro8]

The output impedance of a ladder bridge is so low that it is not suitable for use as a power supply; a ladder bridge is best used for a voltage reference, as when sourcing a comparator (which ideally draws zero current). Simply put, the more current one draws from a ladder bridge, the more the output voltage will sag (towards 0V). Remember Ohm's law!

[thrudd]

D'oH .... you have me there . But with the bridge havinging a smaller efective resistance than the load there is negligible effect, besides, you always design with the load already known right? (joke) A better suggestion by me would have been just as cheap, a zener diode regulator setup. Heck, most cheap (ahem price point) brick dc supplies use the design. Goes to show you how much knowledge you loose when you dont use it. *sigh*

[Doc Holliday]

I'm sure this message was intended to be (dare I say it) coherent. But with ladder bridges, solid rocks and feeding 12 volts into 5, you lost me, although my preferred "flavour "is lemon. I'm still busy chipping away the corners on my new invention: I call it "wheel", but am concerned about lawsuits with the dang thing rolling away. Can you elaborate for us dummies? Thanks!

[cylver]

I believe he was referring to powering the router by either a resistive divider circuit on the 12 volt supply, or by using one of any number of 5 volt regulator ICs, such as the LM7805. This would make it unnecessary to provide separate 12 volt and 5 volt power sources.

[radiorental]

resistor divider = ladder, I thought thats what that was - thanks!

[radiorental]

You'll have to forgive me, I was off sick the day they went over ladder bridge... can you send a link, I couldnt find anything I have a few 24v wall warts lying around, I expect that I can somehow get -12, 0, +5 & +12 out of that, right?

[thrudd]

Ah, I can see why you are not ever getting a zero signal and never max out either. Try a split supply setup for the amp plus a few more passives to tweek the circuit balance. You can dig out a useful circuit from any old Op amp cookbook. You should then be able to get your zero reading at least. The other thing to note when working with mixed op amp circuits is that they are challenging to run rail to rail when you want to (and do when you dont want to) XD

[maestro8]

A split supply is unnecessary to achieve the "zero signal" you seek. An LM358, or any op-amp with a FET in the final stage (such as the 3130/3140) will give you output voltages that are within millivolts of the bottom rail. As for your choice of circuit, an integrator would be a better use for the op-amp. Your current usage could be accomplished just as well with a transistor or two. Furthermore, you don't need the 7404s since most op-amps draw fractions of microamps of current (much, much less than the LED in the router). Build the low-pass filter into the feedback loop of the op-amp to keep its input impedance high (to ensure it draws a minimum of current) and you won't have to worry about overloading the router's output signal.

[radiorental]

thanks very much, when I get around to rev'ing this design I'll get in contact. Didnt want the initial complexity of a split supply... but I now have a few more designs in mind I might do a small production run. Some routers like 5v, the linksys's like 12... so I should maybe create -12, +5 & +12v rails?