Today I  took the wrapping off of and opened up the September edition of Custom PC http://www.custompc.co.uk/ : I’d received it in the post some days, maybe even a week ago; but I hadn’t had time to study it or even look at the cover yet. It flipped open in the “Hands On” section; and there was this article on how to make a PC monitor from an analogue meter.

I have reproduced page 1 of their article. Not intending to break their copyright I have deliberately made it hard to read; yet clear enough to be able to see their circuit and get some idea what the page looks like. I’m afraid if you want to see the article properly you might just have to purchase the magazine, unless they have featured it on their website.

Let’s face it though; there’s probably not a lot of genuinely old-ish analogue meters around; and a decent one can cost anything from a few £s upwards. What’s more an analogue meter is so hard to read if it’s moving at all.

My brain engaged in design mode at this point: Why use a meter at all? Yes it’s maybe nostalgia and “geekracy”, but why not instead have an LED bar-graph? Just for the purpose of being ultra-creative why not make it from individual LEDs rather than a prebuilt bar-graph?

OK easy enough; but first that circuit – Theirs is so basic it sucks:

Their circuit has a signal-diode connected cross-path without anywhere to dump the current passing through it except into the diode’s semiconductor itself: That’s a recipie for disaster if the input signal has a large amount of gain: The signal-diode’s semiconductor material will overload and burn out: In their circuit that diode is connected as a half-wave short circuit.

My remedy is to add a 100 nanofarad capacitor in series with the diode on one side to block any DC component of the signal, and a LED in parallel with the capacitor to indicate any DC component’s presence as well as to act as a partial sink. The greatest current sinkage, though, comes from series pair resistors 6 and 7, which I’ve rated at 1/4 watt just to be on the safe side: Those should sink any remaining AC component of the signal and not get too warm.

Resistor R7 is connected as a current sink and limiter for the entire circuit.

The main deviation from the original design comes where I’ve replaced the analogue meter with 4 resistors and 4 LEDs. It is possible here to have as many series resistor/LED pairs here as you like to a certain extent; bearing in mind the current drain this will cause with respect to the current gain of the input signal.

I really don’t want to get too technical here or I might end up drawing bode plots and nyquist diagrams; whatever.

This isn’t a final circuit, I’ve not built it, and it would be an idea for you, if you’re an electronics hobbyist, to play around with it, adding more LED/resistor series pairs whilst experimenting with the values of individual resistor – Even going into the specialist series with fractions of an Ohm values if you like. Also experiment with the value of R7 and see what effect a higher resistance has on the overall performance of the circuit. An 8 LED bargraph would be excellent and in my opinion would look much more geeky than an analogue meter.

Having looked at this again after publishing I think I might have actually negated the original function of the diode to some extent by removing the DC component from its circuit. Removing or short-circuiting C1/LED1 will reintroduce this DC component back into the equation; in which case now that we have resistors 7 and 8 in circuit they will help sink the DC as well as the AC waveform component, but with the result of a little more heat. (Negligible.) In an ideal world it would be an idea to use two inverting buffers incorporating an op-amp such as a 741; one on each input terminal. These could be powered from the 5V USB supply – But now I run the risk of getting overly technical if I were to go any further; and I see why they used the circuit they did: Not so much built to last as built to work, on a KISS principle: Keep It Simple Stupid. The full redesign required to make this piece of kit perfect as I’d like it to be would need a lot of components, and would further complicate what started out as a simple circuit.

So yes build the circuit and leave out R6 and C1/LED1. It’ll work; but if the input gain goes really high it could end up damaging D1 as I said. Also there’s the added factor that D1 won’t pass a current if the voltage-gain on the input drops really low. (Below Approx. 0.35 volts, which IIRC is the threshold voltage of that particular diode.

OK here’s a better solution: Disonnect D1 and connect it in series with the 10 microfarad capacitor. cathode to its + terminal. Connect another capacitor’s + terminal directly to the common anode of the LED bar display and via an identical series-connected diode with its anode to the – terminal to the in-circuit side, (Bottom) of R7: Now we have two properly polarised capacitors storing the full wavelength.

I’ll alter my circuit diagram for the purpose of clarity. I’ve crossed out the bit of circuit we no longer need and I’ve drawn in the second series diode/capacitor, connected as I said:-

Revised Circuit Diagram

Ok; you get my drift? We added another diode and a second 10uF capacitor; each stores half of the incoming waveform and is kept polarised by its series-diode in both cases: I should have thought of that first time round. There’s no stress whatsoever on either diode now, and the need for a cross-path diode has been eliminated. Consequently this circuit will last much longer because none of the components are stressed in any way.

Forget all previous designs; build this: Experiment with different values for all the resistors, and enjoy the display.