'555' IC Tester

[David G4EBT]

I’ve got rather a lot of 555 ICs of doubtful provenance, and as I've recently been making a few gadgets which use this versatile little IC, I thought it would be useful to make a simple tester to see if I have any duds. Circuits for such testers - often built on strip-board - abound on internet and I claim no originally for the circuit that I used. I’m not a great fan of strip-board so I designed a little PCB on which to build the tester.

The circuit consists of an astable multivibrator, and when the test button is pressed, if the IC under test is good, the two LEDs will flash alternately.

With the component values chosen, the flash rate is about 3 Hz for each LED. (The frequency of flashing is of no importance, and is determined by the values of R1, R2 and C1). The way that it works is that when the test button is pressed, C1 starts to charge through R1 & R2, and when the voltage rises to two-thirds of the supply voltage the flip-flop internal to the IC will toggle, then pin 7 goes low and C1 starts to discharge. When the voltage across C1 falls to one third the supply voltage, the flip-flop resets and pin 7 goes high, then C1 starts to charge up again and the cycle is repeated. If the IC is good, the two LEDs will continue to flash alternately as long as the test button is pressed.

All the bits were from my spares box - another little 'MFJ' project - 'made from junk'! As is my custom for housing my little home-brew projects, I made a little comb-jointed oak box to house the tester. The question arose as to how to devise a test socket for the IC. I just soldered a turned-pin socket into the PCB, then added another five pushed into each other, with the uppermost one glued to the front panel with epoxy glue, and the PCB mounted on stand off pillars below the front panel.

Hardly breaking new ground here, but I thought I’d share it should anyone else ever wish to make a 555 tester.

The 555 is a remarkable little IC, introduced in 1971 by the US firm Signetics, and still in widespread use for such things as timers and oscillators. It’s now made by many companies in the original bipolar form and also in low-power CMOS types ‘7555’. By 2003 it was estimated that 1 billion were made every year.

http://en.wikipedia.org/wiki/555_timer_IC

In terms of longevity, it must rank alongside the 741 op-amp, introduced in 1968, and 2N3055 transistor, which had its 50th birthday in 2014!

Pics of the circuit and the construction of the tester are attached.

In another post I'll add the PCB artwork and component overlay, but it could be built of strip-board if preferred.



Hope that’s of interest to someone.

[David G4EBT]

Here is the PCB artwork and component overlay for the tester.

The PCB 6.5 x 5.5 cms.

[Nickthedentist]

I love your ingenious but practical way of mounting the IC test socket. Far less faffing around than flying leads from the PCB to the socket. And your oak box really finishes it off, impressive stuff.

To the big question now... were any of your ICs actually faulty?

N.

[David G4EBT]

Thanks for your kind comments Nick.

No - none of the forty or so 555s I've tested were faulty.

That said, a couple of years ago I made a little sawtooth generator which specified a 555, which wouldn't work. I etched another PCB and built it again - still it didn't work. A forum chum had sent me the circuit, so I asked if he'd look at it for me. He kindly did so, and substituted the 555 for the CMOS version - a '7555' and it worked right away. So, if a tested 555 seems to work in the tester, I guess there's always a risk that it may not do in some circuits.

As to the boxes, they often take more time to make than the projects that go into them, but as woodturning/woodworking is a parallel hobby it isn't a chore, and it means I'm not constrained to using whatever plastic/diecast/ali enclosures are available, which seems to be fewer and fewer (and dearer & dearer!) these days.

[SurreyNick]

Going off thread, but I would agree with that final observation David. I am currently assembling a few items of equipment and temporarily don't have the facilities to make my own wood enclosures so have been compelled to buy plastic ones. Goodness me, prices are absurd and choice of size so limited!

Lovely little item of test equipment you have made here. As I too have all the components necessary it's been added to my 'to make' list.

Do please keep the ideas coming too, at last count I think I have four of your projects either made or in production!

Nick

[Radio Wrangler]

Just a standard cautionary note about the 555

The output totem-pole stage in all makes of bipolar 555s can and does go unstable on one edge. It doesn't affect light flashers, but if the output drives logic or a counter, expect unintended counts. It also momentarily shorts the power pin to ground for matters of nanoseconds, so you may need decoupling to protect other devices.

David

CMOS versions are OK

[McMurdo]

I made one of these little testers when I first started work with a similar socket poking through the case. Later I added a pot and an output relay so i could use it as a low frequency pulse source for testing or cycling other devices....very useful! Although 555's are only pennies these days, we used to repair some items that had military-grade 555's in them which were about 8 quid each, so well worth testing those!

[paulsherwin]

Originally Posted by SurreyNick

Going off thread, but I would agree with that final observation David. I am currently assembling a few items of equipment and temporarily don't have the facilities to make my own wood enclosures so have been compelled to buy plastic ones. Goodness me, prices are absurd and choice of size so limited!

Charity shops and flea markets often have useful wooden boxes for a pound or two.