UNVERIFIED. This module is drawn and checked in Copper Bottom - the copper agrees with the netlist below, and the checks come back clean. The board it grew from is the Firefly Sequencer, itself unbuilt; rows 0 to 15, columns 0 to 29 of this layout are that board hole for hole but for four parts named below. The rack block has not been built. Every number on this page is a datasheet number, not a measurement. Build it once, fix whatever fought back, and this banner comes off.

Firefly Rack

Eight steps at 0 to 5 V · The Firefly Sequencer plus a rack block · 18 HP · By Onion Madder · Mess O' Pedals

The Firefly Sequencer is a 555 clocking a CD4017, eight step knobs and eight LEDs, a control voltage out and the clock out. In a rack it wants three things it did not have: to run at a control-voltage range other modules expect, to take a clock from outside, and to drive a jack without the eight diodes feeling it. This is that board with all three, on 18 HP.

What is the Firefly, and what is new

Rows 0 to 15, columns 0 to 29 are Firefly_Sequencer.json hole for hole but for four things, each of which the rack block replaces: D9 and the 9V pad (the old reverse-polarity diode - the cut it stood across at (0,26) is gone too, so the 5 V rail runs the whole strip), the CV_OUT pad (the jack now hangs off the buffer), and J5, the one link that tied the 555's output to the 4017's clock pin. Every other net is re-derived from the copper before this page is written.

J5 is now a jack. The 555's output leaves on the CLK_N pad to the clock jack's switch contact, and the jack's tip comes back on CLK_T through 100K (R6) to the 4017's clock. Nothing plugged: the jack's own switch joins the two and the Firefly clocks itself, as before. Plug a rack clock in and it takes over, the 100K limiting what the chip's input diodes see. One link inside the Firefly's own area, J13 from (4,8) down to (15,8), carries the clock node to the empty strip 15 for the resistor - column 8 was the one free column under it.

At 5 V the steps are already right. The step pots divide the chip's rail, so the CV bus runs 0 to about 4.4 V (a diode drop off the top) - the range every rack sequencer speaks - and CLK OUT is a 0-to-5 V gate. Half of the TL072 follows the CV bus and drives CV OUT through 1K, so the eight diodes and the 100K pull-down never see a rack input's load. The other half follows ground.

The LEDs will be dimmer at 5 V. R5, the one resistor all eight share, is 2K2, and that is about 1.4 mA now. It is the Firefly's value and it stays on this page; 1K would suit. Its R3 and R4 lie flat across two holes, which Copper Bottom warns about - that is the way the Firefly draws them and it is fine.

The layout file is the guide

As on every stripboard guide here, the build document is the file itself: ↓ Download Firefly_Rack.json - import it into Copper Bottom and the editor gives you the board picture, the placement walkthrough, the electrical checks and a printable build sheet, all from the same file. No coordinates are written out on this page on purpose: the editor is the viewer, and a number typed twice is a number that can drift.

The netlist

Quoted verbatim from the layout file. It was written from the board, so checking the board against it can only pass - what it is for is carrying the circuit somewhere else, and telling you what each strip is supposed to be doing while you stare at it. The bug board's nets keep the names they have on its own page.

The off-board wiring

39 pads leave the board. Ten of them are the power header.

The panel

18 HP, which is 91.4 mm of a 128.5 mm blank - Amplified Parts sells reversible black-and-aluminum blanks in every width from 2 to 20 HP. The template is drawn for 9 mm Alpha pots (7 mm holes), Thonkiconn jacks (6 mm), sub-mini toggles (6.5 mm) and 5 mm LEDs, with the rack's rails kept clear top and bottom and the four M3 mounting holes where Doepfer puts them. I/O along the bottom, controls above, as everything on this site does it.

The rack block, the same on every module

The right-hand end of every board on this shelf is the same idea drawn six times. The 2×5 header takes the rack's power cable, mounted with its pins running down the strips so each pair of pins lands on one strip: -12 V, then three grounds, then +12 V. That is the only way it can go on stripboard - turned the other way, the -12 and ground pins would share copper. The red stripe on the cable goes to H1 and H2. A 1N5817 stands in series with each rail so a cable put on backwards does nothing instead of everything. A 78L05 then makes a 5 V rail for the chip, and a TL072 on the real ±12 V does the rack-level work.

Five volts for the CMOS, not twelve, is what makes these rack-polite for free. A CD4000-series input switches near half its supply, so a +5 V gate from a sequencer clocks a 5 V chip directly but would never reach the ~7 V a 12 V chip wants; a pot across 5 V makes exactly the 0 to 5 V control voltage every rack sequencer speaks; and a hot or negative rack signal into a CMOS input needs only a 100K in series, the chip's own input diodes doing the clamping at well under a tenth of a milliamp. It is also a change from the 9 V these boards were built at - every chip is rated down to 3 V so the circuits are fine, but the oscillators will sit at slightly different pitches, and that is one of the things the bench is for.

The IDC socket on the power cable overhangs the header by about two strips at each end. Nothing tall stands there - the generator that drew this refuses to put anything standing under the socket - so keep it that way if you move a part.

How it works, in one breath

Nothing about the sequencing changed: the 555 ticks, the 4017 lights one output at a time, each output feeds a pot, and the wipers come back through diodes onto one bus so whichever step is live sets the voltage. What the block adds is a buffer between that bus and the world, and a jack in the clock line with a switch in it. The switch is the clever part and it is in the jack, not on the board: a Thonkiconn's normalling contact touches the tip until a plug pushes it off, so the internal clock is what the tip carries until something else is.

Things to try

• Play the Dragonfly. CV OUT into its 1 V/oct jack: with 4.4 V of range that is four and a bit octaves of sequence, and the scale trimmer decides how the steps land
• Play the Cicada. CLK OUT into a gate jack turns one voice into the rhythm the LEDs are showing
• A reset jack. RST through its own 100K to a jack lets a rack gate restart the sequence - one resistor, one pad, not drawn
• Brighter. 1K for R5

The Firefly is the Firefly Sequencer layout from this site - the Baby 8 idea, a 555 and a 4017. The normalled clock jack is how every rack sequencer with an internal clock does it. Drawn in Copper Bottom.