Zero-Electrolytics 555 Timer

The more resistance the better

A timer circuit that turns on a pump for a short time every five minutes can be easily made using the famous 555 IC. Unfortunately, it's almost impossible to avoid using electrolytic capacitors to achieve such a long period, which makes the circuit less accurate and reliable, especially in the longer term. However, there is one solution to this problem, and that is to use a special 1 giga-ohm resistor!

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Materials

130257-PCB.pdf

Gerber file

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PCB

Download Gerber file: 130257-1.zip

Component list

COMPONENT LIST
Resistors
R1 = 1GOhm 0.25W 10%; TE Connectivity type RGP0207CHK1G0
R2,R3 = 10MOhm
R4 = 1kOhm
R5 = 10kOhm
Capacitors
C1 = 390nF, lead pitch 5mm or 7.5mm
C2 = 680nF, lead pitch 5mm or 7.5mm
C3 = 100nF, lead pitch 5mm or 7.5mm
Semiconductors
D1 = 12V 0.5W zenerdiode
D2 = 1N4007
D3,D4 = 1N4148
T1 = IRF1405ZPBF
IC1 = TLC556CN (DIP14)
Miscellaneous
K1–K4 = spade terminals, Faston, 0.2’’ pitch
JP1 = 2-pin pinheader, 0.1’’ pitch
S1 = slide switch, miniature, 1 C/O contact; C&K Components type OS102011MA1QN1
S2 = pushbutton with make contact, 12V 50mA, 6x6 mm
F1 = fuse, 10A fast, 5 x 20 mm, with PCB mount 15A holders; 2 pcs Cooper Bussmann type 1A3399-10-R
PCB # 130257-1

Discussion (0 comments)

Paul Gallagher 2 years ago

I ran into an issue attempting to reproduce this circuit - could not get it to work at all with 1GΩ resistors.

After further investigation I was successful, but only after recognising that the very low discharge leakage specification of the TLC556C is critical (200x lower than most other 555/556 components).

This is not pointed out in the article, leading one to perhaps assume that any 555/556 functionally-equivalent component might do. If the article is ever revised or re-published, I'd recommend to specifically emphasise the critical max discharge off-state current specification of the TLC556C.

All the details of my testing the circuit and finally achieving success are covered in my project #631 notes:
https://leap.tardate.com/electronics101/555timer/longrunning/

0 Comment(s)