This circuit requires a digital voltmeter panel, you can get them on ebay for around £4-8, depending on where you get them from and what you want. Bear in mind you will want to mount it in an enclosure, so getting a panel with a bezel is a good idea as it will sit nicely. Otherwise you may have to mount it inside the enclosure, in which case you will probably want a window, which can be done using some thin transparent perspex, but is a right P.I.T.A.
The output is around 1.6v-9v depending on your input supply, the minimum voltage set by R2. On the pedals I’ve produced I used a 680r as R1, which is fine, but 2.2k cover you better. You could put caps on the input and output of the 7805, something like a 0.33uF tying the input to ground and a 0.1uF tying the output to ground should work OK, and just helps to stabilize the voltage, but it will work well without it.
Make sure you use a linear pot for the VR1, as an audio taper would be very strange here! For the output I used a jack rather than a socket, mainly because it stops people plugging into the wrong side. If you want reverse polarity protection then you could put a 1N4001 or similar in series with the input voltage, but this will reduce the output voltage by around 0.6v (off the top of my head), so bear this in mind. Depending what voltmeter panel you choose it may have reverse polarity protection built into it. You shouldn’t need a heat sink on your 7805, but if you find it getting too hot then you can connect it to you enclosure to help dissipate the heat, providing you’re using a Hammond style enclosure. I used a 1590A, and it is a tight fit, but you can get everything in there, even if you have to use a shoe horn!
D1 3mm LED
R1 2.2k 0.25w
R2 2.2k 1w
U2 Digital Voltmeter Panel
VR1 10k Lin
As you can see, it’s a pretty basic circuit, even if you add the mods I have described above. The hard part is firstly sourcing a good panel, and then squeezing it into the enclosure. I’m probably not going to make any more of these, at least for the foreseeable future, as I don’t have the facilities to cut the rectangular hole required for the panel efficiently enough to be covered by the price I charge for each unit.