A week or so ago, I threw out for comment a couple of hypothetical oscillator circuits (Pierce) using the inverter property of the 555 and a low-frequency crystal. I did not have a crystal at the time, but got a few cheap 32.768 KHz crystals yesterday.

With some slight modification, both the 555 and the CD40106 inverter circuits worked. The circuits were done on a bread board, so the values may be not quite right for a real design. Only one capacitor is required. Both give close to 50% duty cycle. The CD40106 is a nice square wave; the 555 is a square wave with a slightly wavy top, like a question mark on its side. The CD40106 starts quite quickly; the 555 takes a little time, maybe a second or two when starting cold. Frequencies were measured using a Racal Dana 1992 counter and were stable as shown. The 10E-6 place was ±2

Any comments? I don't recall seeing a crystal-controlled 555 before. Has anyone else? My experience is very limited.

John

 

Looks like an entry into the 555 contest to me.

I might just steal that sucker for the 555 articles, unless you object.

 

Unfortunately, the contest is over. Now I am just a day late and out another dollar.

You are welcome to use it. My main concern is that the 555 version is working because of some artifact. Be sure to verify you can get it to work too. These are the crystals I used: http://cgi.ebay.com/ws/eBayISAPI.dl...91484&ssPageName=STRK:MEWNX:IT#ht_2087wt_1242 . My 555 is the NE555.

If it doesn't start, increase the capacitance a little or briefly touch it. The 8 pF version did start by itself after 8 hours of sitting cold, but it did require a little longer than after a brief stop to lock in on the frequency.

John

 

Would it be possible to use this bit of circuit to make a 1Hz clock oscillator? The 555 part.

 

You're going to need a ripple counter, similar to the 4060. A 4040 would do it, at least down to an accurate 2Hz.

 

OK, thanks. I'll see what I have.

 

A 4060 would also work, since it has a 14 part ripple counter.

 

Would it be possible to use this bit of circuit to make a 1Hz clock oscillator? The 555 part.

You can also use a 8 pin PIC MCU and a clock crystal. For that task.

 

Yes, I know. I got a 12c-something which I think is One Time Programable. How accurate would that be? Dead on, or drifting.

I would like to try this without any programming. I will do this during the weekend.

 

Yes, I know. I got a 12c-something which I think is One Time Programable. How accurate would that be? Dead on, or drifting.

I would like to try this without any programming. I will do this during the weekend.

yes the PIC12C has an EPROM. But no window. So it is OTP. But if you are like me you do only need 1 try to get a program correct(well..ah.hmm..). Elfa do have 10 types of 8 pin PICs with flash. Some of them are just like an 16F628 with only 8 pins. The drifting will depend on your crystal. Given stable conditions

 

Do you have an accurate frequency counter? I only built the circuit on a breadboard and fear that a real PCB design would require a different capacitance. As you know, the capacitance loading can pull the frequency in either direction. If too much, the freq will be slightly low; too little it will be high.

If you have a CMOS 555, it might be worth a try. Maybe I will try it after breakfast.

Edit: Breakfast will wait. Yes, the CMOS 555 works much better. It starts right up and has a nice square wave without the little hump. Uses same capacitance as the NE555 on the breadboard.

John

 

I got this frequency counter.

First I thought I would try it on a breadboard, but now I think a perf-board. I have the LM555 and the NE555 and some crystals of different values. We'll see when I get home from work....

 

Remember, with the 555, it won't go very high in frequency. The CMOS version I tried was the LMC555CN. With 8 pF it is at 38.767910 ±2LSB. John

 

Yes, I know.

I will try to squeeze it down to 1 or 2Hz.

 

The 555 version might start more quickly if you used a 330 Ohm pull-up resistor on the output.

 

That does it and fixes the square wave. Nice to know that. Thanks.

John

 

The Darlington follower on the output of the BJT 555 causes a ~1.3v drop from Vcc. With 4.8v as Vcc, your output only gets up to around 3.5v, which makes it take a long time to get to the 2/3 Vcc trip point. The 330 Ohm pull-up helps shorten that time considerably. A CMOS 555 wouldn't have that problem.

As far as the 40106, you could substitute a 4093 quad NAND Schmitt trigger gate, tying the two inputs of one gate together and using it as an inverter.
Don't forget to connect any/all unused inputs to Vdd/GND

 

There are lots of ways to skin a cat. For this little challenge, I relied on the application notes from Fairchild and others. My main purpose was to see if it could be done with the 555, as I had not seen any examples of that application per se. I keep neglecting the output differences between 555 versions. With the bench experience gained from this experience, I am unlikely to forget that again.

As for its practicality, I don't envision much, except the thrill of being able to say a 555 was used. For the price, it was worth it.

John

 

What happened to the schematic?

Just got a 32.768KHz crystal and was about to test it, but I can't find the schematic anymore.

 

I'd like it too, but I can almost draw it from memory. I'll give Jpanhalt first chance, then I'll fake it. I have a decent memory for circuits.