Schmit trigger help

[axeman22]

Hi,

After a little advice on Schmit trigger IC's. I have a SN74LS14N chip. I've got a signal which is relatively square wave at about 100mV Peak to Peak. I want to take this circuit and make it bounce nice and hard with solid levels in accordance with the input voltage. So a +50mV input would equate to around +5v and a =50mV would equate to -5V. So I'm thinking this is ideal for a schmit trigger circuit?

I understand(perhaps incorrectly) that a schmit trigger will take the input to either the Vcc voltage or to ground depending on the input. - is that correct..? with the IC I hve, what are the trigger voltages which will result in the schmit trigger transitioning from state to state on the output. I read the data sheet but it's a bit complex for simple old me.. here it is : http://www.datasheetcatalog.com/data...N74LS14N.shtml

appreciate if anyone could confirm my understanding of the schmit trigger.. and if anyone has other ideas on how to achieve the desired output based on the inputs - a fluctuating AC signal (decoded POCSAG code actually) of around 100mVp-p. I'm trying to take this small signal and drive it up to RS-232 levels, whilst cleaning it a little at the same time.

Thanks!

[Bill_Marsden]

What is the max frequency?

[SgtWookie]

I'm afraid that your 100mV p-p signal is not of sufficient amplitude to trigger a 74LS14.

You really need to use a comparator with hysteresis; which is basically a Schmitt trigger.

Have a look at the attached schematic & simulation. The title is somewhat misleading; with Vcc=5v, hysteresis is actually about 25mV. To increase hysteresis to 50mV, decrease the value of R5 from 470k Ohms to around 240k Ohms. If you're going to be using a Vcc of 10v, then leave it at 470k.

C1 blocks the DC level of your 100v p-p signal (A), shown on the simulated O-scope on the bottom as the cyan trace.
R1/R2 are a voltage divider that centers your signal at Vcc/2; in this case 2.5v.

R3/R4 centers the voltage reference at Vcc/2. In actual practice, R3/R4 should be about 4.3k Ohms, and there should be a 1k pot between them to allow for adjustment; real-world resistors are not exactly their specified value.

R5 provides negative feedback to the reference input for hysteresis. This also keeps the circuit from oscillating on it's own.

R6 is a pull-up resistor; this is necessary because the LM339 has open-collector outputs. It can sink current, but it cannot source current. R6 provides a current source.

[axeman22]

my god.. that little program just makes it look too easy! Circuit Maker.. I just had a look and it's no longer supported.. ?

I had been using SuperSpice.. seems very powerful but I find to tooo complex for simple stuff like this.

what would the advice be for a person like me who is just playing around with circuits for the fun of it and the learning - as far as a electronics sim program like this goes? I had done little research but ti seemed to me there was no clear leader in the field. if possible I'd like also to be able to have a Picaxe micro in there - do any of these circuit sims handle having a microprocessor in there well?

[SgtWookie]

Originally Posted by axeman22

my god.. that little program just makes it look too easy! Circuit Maker.. I just had a look and it's no longer supported.. ?

Correct. I just use it for "quick and dirty" schematics/simulations for this site; the interface is pretty intuitive, and I can throw together a simulation more quickly using it than other simulators. The library has most components that a typical hobbyist would use. But it's at least 12 years old, and can't be updated; you can't add to the library, use macros, or use more than 50 components in a schematic.

I had been using SuperSpice.. seems very powerful but I find to tooo complex for simple stuff like this.

Hadn't heard of it. There are quite a number of simulators available, costing anywhere from nothing (freeware) to tens of thousands of dollars per installation.

what would the advice be for a person like me who is just playing around with circuits for the fun of it and the learning - as far as a electronics sim program like this goes? I had done little research but ti seemed to me there was no clear leader in the field. if possible I'd like also to be able to have a Picaxe micro in there - do any of these circuit sims handle having a microprocessor in there well?

Some do.
Circuit maker was superseded by Altium Designer. Circuit Maker Student is/was a freeware download. Altium Designer cost around $10,000 per installation a few years ago.

Microchip's MPLAB IDE has a built-in emulator for various PIC uC's. You could test out a program in it to see how it'll work. You could use another PSPICE program to test out the linear portions of the circuit.

Linear Techology's LTSpice is a very good (and free) PSPICE and schematic capture program. The interface is just a little bit quirky, and the library contains mainly just Linear Technology's components. However, you can add as many components to the library as you wish.

[SgtWookie]

Oh, one more thing - I used an LM339 quad comparator because you can find them at your local Radio Shack. There are 3 other comparators in that IC; if you aren't going to use them, you should ground their inputs, otherwise you may have stability problems.

[axeman22]

well I found a download of Circuit Maker... but it won;t install under W7! .. I've just spent 20 minutes trying to find out hwo to get it working.. but it seems no go :-(

having a play with a program called MicroCap at the moment.. hmm... wish I just could just pay $29.95 once of and get a 'real good' program that solves all my needs.. this is a new area to me(sims), there seems so many options.. I just want to build circuits and get going.

if I may ask..SgtWookie how would you take the circuit you made and make it trigger on 100mVp-p fluctuating about zero V and make the output around 5Vp-p fluctuating about zero V also..? (being a bit lazy here but you're so damn quick!)

[SgtWookie]

Originally Posted by axeman22

well I found a download of Circuit Maker... but it won;t install under W7! .. I've just spent 20 minutes trying to find out hwo to get it working.. but it seems no go :-(

I am NO fan of Vista/W7! I like W XP Pro just fine, thxvrymch.

having a play with a program called MicroCap at the moment.. hmm... wish I just could just pay $29.95 once of and get a 'real good' program that solves all my needs..

Don't say that too loudly - the Chinese may just do that!

this is a new area to me(sims), there seems so many options.. I just want to build circuits and get going.

There certainly are quite a few options out there.
Have a look at Tina-TI over at Texas Instruments' website. http://www.ti.com It's a free download. It's a somewhat stripped-down version of Tina.

if I may ask..SgtWookie how would you take the circuit you made and make it trigger on 100mVp-p fluctuating about zero V and make the output around 5Vp-p fluctuating about zero V also..? (being a bit lazy here but you're so damn quick!)

Almost like I sorta knew how to fake it pretty well?

OK, do you have a -5v rail source?

[axeman22]

No.. I think for the moment in testing I'll just use batteries.. so it will be +4.5 and -4.5V rails actually. I need to drive RS-232 ok and the RS-232 spec says that between 3-15 volts is ok so 4.5 should be fine I'm thinking, and easy enough to get a hold of .. going to download and try that program now.

[SgtWookie]

OK, I put in a -4.5v Vee, some Zener diodes in the output to limit the output voltage swing to around +/-2.5v, and the cap/resistors on the input were no longer necessary. The Zeners really aren't necessary, as RS232 could swing to +/-12v.

Note that the output polarity is opposite from the input.