It's really a little worse than that; the specs on the trip points of that (74HC14) chip are VERY loose. As others have stated, any of the CMOS '555 chips would suit this application. The trip points for a NE555 (or ICM7555, or TLC555, or TS555, or LMC555) are at 1/3 and 2/3 Vcc, accurate to a few percent, and the same resistor/capacitor negative feedback circuit as for the Schnitt trigger makes it oscillate. You just leave the extra pins open...
The sensitivity to input voltage of each 74HC14 chip (and indeed, of each section of each chip) are spread over hundreds of millivolts; the data sheet only offers (at 4.5V) a range of 2.3 to 3.15 volts for HIGH, and 0.9 to 2.0V for LOW.
Logical models instead of models suitable for analog simulation. PSpice has the same problem, and I'm fixing that in my next round of model releases. ...Jim Thompson
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| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
On a sunny day (Fri, 21 Mar 2014 21:02:43 +0100) it happened Cursitor Doom wrote in :
OK let me explain something about GM tubes. In a simple way.
It is just like a neon that has not ignited (gas filled tube). When ionizing radiation hits it it ignites and a fixed voltage will appear (drop) over it (just like a neon can be used as 'zener'). Here is the basic circuit diagram of a GM counter input stage:
High voltage, for example 400V |
500 MOhm current limiting resistor R1 | |--||-| C1 capacitance to ground of tube + HV line | GM tube (drops some 200V or there about when 'on') |-------------------------------------------------------->output sense resistor | ///
So lets say the 'on' voltage of the GM tube is 200V. Normally, without radiation no current flows (tube is insulator).
Without C1, and when triggered, the current is 400 - 200 / 500 MOhm, so say 200 / 500 uA or .4 uA If the sense resistor is 100k Ohm it causes a 40mV peak.
With C1 taken into account the impedance is much lower than 500 MOhm, and in fact, given that the impedance of the GM tube when conducting is extremely low, C1 discharges into the sense resistor. My measurements (with a 10 MHz analog scope) show pulses (try a dark room) of about 1 to 2uS wide. The whole secret is in C1 to get reasonable output, I use a short piece of coax (a few cm) as HV capacitor. So whatever you drive, it should be able to handle the C1 discharge current, I drive it via the RC network I did draw the ASCII diagram for in my first geiger counter, and in the latest one directly into the base of an NPN. The C1 should be big enough to win from the storage time in the base. I'd think zeners have high capacitance and suck here (you get a capacitive divider) transistors do just fine.
So the whole thing is extremely simple if you understand the function of C1 Hope this helps. One make GM tube or the other makes Ferry Little difference. Look at the tube specs for the plateau voltage for what appears over the tube when it is 'lighted', Make your HV so it does not light without any radiation already. Set C1 to the minimum needed to operate your input stage, and you get a long tube life I would think. Extremely high HV makes no sense, C1 is the key.
There is more to it, the tube needs to go 'out' too, that is why R1 is so high. And more...
Yes, when Jim asked about this I posted a revised diagram which included it. If you can't find that posting, it's just a 3.9v zener connecting the input of the first Schmitt gate to ground.
There's nothing flaky about this circuit. And if you're going to make inaccurate observations, please don't compound the sin by referring to the wrong chip!!
We seem to be going way off on a tangent here. I only asked for suggestions as to how to drop a few tenths of a volt. The circuit works FINE at 4.5v but not at 4.8v (despite the chip being good at up to 7v supply according to the datasheet) Can we please just focus on this original question, which has all the necessary info in it already.
Don't mind Larkin... Larkin is just S.E.D's resident "It's all about me! Me! Me! Me!" manic depressive. Has to throw stones, and is off on a tangent not understanding what your problem really is.
What I'd recommend would be something along these lines....
Twiddle the resistors and capacitor to suit your conditions (get a better scope ;-)
Some hints:
If LTspice is like PSpice, simply loading a logic element with a resistor will change it from outputting 1's and 0's and instead produce +5V and 0V.
And I just discovered that I can replace the $G_DigPwr call in the PSpice 74HC14 model with simply the label of my local supply, VDD, and it will track that voltage... so thanks for the question, I learned a new trick today!
Have a good weekend, everybody! ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
It defaults, like PSpice, to 1's and 0's, you know... binary ;-) ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
only draws power during a pulse (except for leakage), so your battery ought to last quite awhile. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
drive the gate like your driving the second stage, just put the R and cap together in parallel to the gate and common. The drive can also be used to avoid back bleed.
At least it reduces it down to hardly no components and it will work.
Since the power line of mosfets has a lot of charge at the gate, you won't need much added Cap, maybe not even any..
Many thanks for the circuit, Jim; looks promising. I know it's a total PITA trying to design for something like this where so many important parameters can only be guessed at, so respect is due you for attempting it at all.
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
I haven't read the whole thread, but here we are, about 60 posts downstream from your original post, and you still keep asking for a band-aid solution to a disease you can't identify, for a terminally ill patient.
Referring to Fairchild's app note, it seems obvious to me that as Vcc increases, Vt+ of the Schmitt trigger and the drop across R will also increase when the first inverter's output goes low.
Then, as the drop across R increases, the increased current through the output totem-pole of the first inverter will also rise.
That, with the drop across the diode, could make it impossible for the input voltage to the Schmitt trigger to drop below Vt+, forcing the output to remain perpetually low when some Vcc is exceeded.
Two easy band-aid solutions would be to use a Schottky for the diode and to increase the value of R, but if you're going to go that way I'll have to stop eating canned fish.
Why not use a 7555 or a couple of comparators and make the circuit rock-solid, yes?
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