Why do some people post a schematic as a WORD DOCUMENT??
A letter to somebody is a Word Document.
A PNG file type is a schematic.

 

sorry about that

 

I just got a heap of components, will start building tonight hopefully.

 

Here's how I would do it...

IMO, you really do need a voltage regulator. The selection from the parts list sucked, so I home brewed one.

Remember what I said about using diodes for logic gates? CR1, CR2 and R11 are an OR gate.

 

nice one Bill...

dougal

 

That does look good Bill.

I just purchase a power supply yesterday that can switch between 9,12,15,18,20,24 VDC so that takes out the need for regulated power supply.

For the voltage divider, does placing resistors in parallel make it more accurate?

 

You can use resistors in parallel to get closer to the desired resistance/voltage than you could easily otherwise (without using a pot).

Here's an online parallel/series resistance calculator:
http://www.qsl.net/in3otd/parallr.html
Bookmark it for future reference; a handy page to have.

One thing missing from all the schematics is a 220uF cap across the 555's Vcc and GND terminals. It should also have a 0.1uF cap with very short leads across them. When the 555 changes output states, it briefly shorts out the supply. The caps help a great deal to eliminate transients on the supply.

It would also be a good idea to place small caps (say, 220pF to 10nF) from the junctions of the resistive dividers to ground. Resistors create noise, much as a nozzle on a garden hose. A small cap from the junction to ground is equivalent to throwing the nozzle in a bucket of water; makes things nice and quiet.

 

I've been thinking about it, I have to disagree with the 220µF cap strongly, the pulse Wookie is talking about is measured in nanoseconds. An electrolytic of this type doesn't even come close to this speed range. The nanosecond pulses do leave a mark, but for a circuit like this none of the extra capacitors are that needed. I have built huge numbers of the hysteretic oscillator with no problem without any extra caps, and they have never failed to perform to theory.

This is a breadboard demonstration circuit, I doubt you could tell the difference either way. If it were sharing fast digital logic I would agree with the small bypass capacitors, as the LM339 is a pretty fast chip for its class, but as is this is mostly a slow DC type circuit built for demonstration of concept.

I have seen circuits where it mattered a lot, usually with 555 oscillators running at similar frequencies. They tend to sync in with each other if allowed. If pulses like this were allowed to hit digital gate ICs it might cause spurious pulses in the system. Generally in circuits like this all the digital chips (555's included) get a 0.1µF cap next to them on the power supply.

It is something you need to be aware of, but also judge the application.

 

hi marx hope things are going ok,i never got on too well with breadboards, i might have got a bad one though.

i think bill was right about the voltage reg, especially with plug in power supplies, did a quick check last night, even a 1 volt variation on the power supply, resulted in over half a volt change on the reference voltages.

i finished the circuit on the lines you outlined a few days ago, but as bills was so far superior, i didnt bother putting it up, especially as i got shouted at...
but we can learn just as much from what we do wrong, so any and all comments, most welcome,

dougal

 

I needed a break so I've been fishing. Gonna have to get my A into G this weekend.

 

Hey Bill in your circuit wouldn't the 555 only be reset when voltage goes above 14V? making A go high

And don't we need it to be reset as soon as input voltage comes back over 10V making B go low.

 

Reset in this context is on/off. If the either of the comparitors go high the 555 will be on, either flashing or LED on. It is possible the voltages won't be great enough, but I think they will. If the comparitor output is high it will be at least 9V on pin 4 on the 555.

Comparitor c goes high if the input is above 14V, Comparitor d goes high if the input is below 5V.

The circuit is regulated to appoximently 12V via the onboard regulator.

 

I got to thinking about it, so I ran an experiment. A 555 powered by 9VDC will stay on with anything 0.8V or higher on pin 4, so my circuit would have no problem. While it is a digital input, it isn't part of any digital family. I suspect the closest family would be DTL. If you have enough voltage to bias a transistor on, you have enough to turn it on.

 

I've made some revisions to my circuit. I have a regulated power supply of 18V at 1.2A.

 

Here's how I would do it...

IMO, you really do need a voltage regulator. The selection from the parts list sucked, so I home brewed one.

Remember what I said about using diodes for logic gates? CR1, CR2 and R11 are an OR gate.

Bill another question about this circuit. With the supply rail connected to the output of the LM339 wouldn't the circuit draw power from the supply rail regardless of the comparator being high or low?

I'm struggling to come up with the appropriate resistor value for my circuit mainly in regards to fully saturating the transistors..... I'm not even sure if the LM339 sink voltage/current is enough to saturate them. The transistors I have are BC558s.

Actually I'll put the names of the components in my schematic now

 

 

hi marx, while youre waiting for bills reply, if youre going with 18 volts this could be too high for the 555(spec), you can drop it locally without affecting your divider calculations by chaining a couple of diodes in the 555 supply .7volt per diode.
(or an led, 3 volt drop, but not sure about this,)

 

Bill another question about this circuit. With the supply rail connected to the output of the LM339 wouldn't the circuit draw power from the supply rail regardless of the comparator being high or low?

I guess Bill's not available, so I'll try to answer your questions.

The LM339 quad comparator has open-collector outputs. This means that their outputs can only sink current, they cannot source current. The 2.2k pull-up resistors are there to provide a source of current.

When the comparator outputs are low (sinking current), there will be about 8.2mA current flowing through the 2.2k resistors. When the comparator outputs are high, there will be a good bit less current flow, as there will be a voltage drop across the two diodes, and R12 will limit the current through Q2's base.

I'm struggling to come up with the appropriate resistor value for my circuit mainly in regards to fully saturating the transistors..... I'm not even sure if the LM339 sink voltage/current is enough to saturate them. The transistors I have are BC558s.

Those are PNP transistors. You generally can't substitute a PNP transistor for NPN transistors unless you "flip" the entire circuit upside-down; which you wouldn't want to do.
Do they have an A, B, or C suffix? If so, that categorizes their gain (hFE)

 

The other guys covered the explination (for which I thank them). LM339 (and a lot of other comparitors) have open collector, which as far as I know started with digital circuits. You can tie two open collectors with one pull up resistor together, it is directly equivalent to a AND gate, which can be useful.

Usually LM339 uses a 10KΩ resistor for pullups, but since I was using that value for base bias (and that was on the high end for switching those transistors) I lowered the pullups needed for the LM339 a bit.

Basically I was trying to meet the current specs given in the design.

You circuit uses the Base Emitter junction as part of the pullup, which is OK. When the open collector transistor in the LM339 turns off the transistor gets no current, an off condition.

 

'if youre going with 18 volts'

sorry, missed your 9v divider,i'll have to get a better monitor,

dougal