Tiny Transistors' Clever TT555 Is a Pin-Compatible 555 Timer Built from Discrete Components

Packing 26 transistors and 16 resistors into a 10x10mm PCB, the TT555 is based on Hans Camenzind's original circuit design.

Pseudonymous maker Robo, of Tiny Transistors, has taken on a new challenge: Creating a functional 555 timer out of discrete components, taking up no larger a footprint than the original dual in-line package (DIP) integrated circuit version.

"After dissecting a whole batch of 555 timer ICs I thought it would be an interesting project to make a working copy of the 555’s internal circuit out of discrete components, in the same physical space as the original IC," Robo explains of the project. "I used the original circuit as designed by Hans Camenzind, as a tribute to his design but also because it uses the smallest number of transistors among all different designs I found while dissecting the various 555 ICs. Discrete transistors are larger than discrete resistors, so the original design saves space compared to newer versions that include several more transistors."

"The total bill of materials for the TT555 comes to 26 transistors and 16 resistors. I’ve added a 100 kOhm base resistor to Q25 to protect it from damage, because most transistors cannot withstand more than about 1 V across their base-emitter junction. The lateral PNPs used in integrated circuits are actually very poor transistors with low gain and a high base resistance, which can however withstand rather high voltages across their base-emitter junction without breaking down."

Designed in the open-source electronic design automation package KiCad, the TT555 is crammed into a footprint of 10x10mm (around 0.39x0.39in), onto which a number of extremely small components are hand-soldered. "Building on my experience assembling the TT741," Robo writes, "I had developed a feel for the proper amounts of solder, flux and heat needed to correctly solder these tiny DFN1006 packages."

The finished articles are slightly larger than their integrated predecessors, though only just — and match the originals for pin spacing, making them drop-in replacements. Tested in astable mode on an oscilloscope, the TT555s proved worthy - generating a clean 670 kHz frequency. That should come as no surprise: Robo's been here before, having designed a compact discrete replacement for the 741 op-amp.

The full project write-up is now available on Tiny Transistors.

Gareth Halfacree
Freelance journalist, technical author, hacker, tinkerer, erstwhile sysadmin. For hire: freelance@halfacree.co.uk.
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