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random output

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George L.

New Member
hello,

I am making a little "project," that requires a random output...example....
I have two motors, a capacitor gets charged and I would like a very simple
circuit (555IC, transistors, resistors, and capacitors) to randomly discharge the capacitor to one of the motors about once or twice a second.

The device should be as simple as possible because it is supposed to be smaller than a cubic inch, (part to part soldering)

I would really apreciate some help

thanks in advance,

George L.
 

panic mode

Well-Known Member
you need better definition of signal type...
 

George L.

New Member
I don't know what you mean by signal type....

A 3300 uF capacitor is charged by a small solar cell and the cap should then randomly discharge to one of two motors. I have no Idea how to make a simple circuit that would alow for this function.
:?
George L.
 

panic mode

Well-Known Member
Hi George,

all i wanted to say is we might need more info. for example it could be analog signal (like noise where signal amplitude is random) or digital (same amplitude but changing duty cycle for example). is this some sort of bug randomly crawling around?
 

George L.

New Member
Hello panic mode,

thanks for your help. yes it is a bug creature that is randomly crawling around, you guessed it exactly!

Instead of the usual light following bug, I wanted to make one that just jumps around randomly.


In the bug there are two of these identical circuits running on only one cap.

The basic circuit without the random feature which would dump the electricity to one of two motors when the capacitor is charged...I would like the cap to randomly discharge to one of two motors.


I would really appreciate if someone could help me make a circuit to do the random part.

thanks,

George L.
 

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Roff

Well-Known Member
I think that an oscillator will have enough randomness to enable one or the other of the two circuits randomly IF both circuits have the same thresholds. Unfortunately, I suspect that one will almost always fire first because their thresholds are not identical. Threshold variations will be due to Vbe and LED voltage mismatches, as well as beta variations.
I'm posting a schematic which you can think about or play with. Maybe you can think of some way to solve the problem, if indeed there is one.
 

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George L.

New Member
thanks a lot Ron H, I really appreciate your help.

I don't have a 74HC14, But I have a 74AC240, I think I could also use that not sure though....

I will give your circuit a try....

I am pretty newbie with gates, could you please explain your theory...thanks alot

George L.
 

Roff

Well-Known Member
74XX14 is a Schmitt trigger. A Schmitt trigger has hysteresis. Google "Schmitt trigger" and "hysteresis" if you need more info. The circuit I posted is an oscillator, and the inverters ("gates") must have hysteresis, or the circuit will not oscillate. There are other Schmitt triggers you could use, but it is important that they are specified to work down to 2 volts. 74HC14 is one of the common ones. You might be able to use 74AC14, 74AHC14, etc. I haven't checked their specs.Here is a good article about Schmitt triggers.
The circuit you originally posted works by charging the cap up until the voltage is high enough to allow current to flow through the series combo of the base of the PNP and the LED. When this occurs, the PNP starts to turn on the NPN, pulling even more base current through the PNP (via the resistor), which is a regenerative process. The collector-emitter voltage of the NPN rapidly goes to near zero, activating the motor. The high motor current rapidly discharges the capacitor, and its voltage goes so low that the regenerative PNP-NPN switch turns off, and the process starts all over.
My idea is that, somewhere during the capacitor charging cycle, the oscillator will start (probably around 1.5V). It will rapidly connect the cathodes of the LEDs t0 GND alternately. The frequency of the oscillator, the time when it starts, and the time when the cap voltage reaches the regenerative threshold will vary depending on incident light, so in theory, either circuit could be enabled at the time the threshold is reached.
As I said, this may not work due to threshold mismatching between the two circuits.
 

panic mode

Well-Known Member
Make sure you know that voltage from solar cell will vary depending on light conditions etc.
74HC family can run on 2-6VDC power,
74HCT requires 4.5-5.5VDC,

I think I would try 4000B series chip like 4093B etc.
 

Roff

Well-Known Member
panic mode said:
Make sure you know that voltage from solar cell will vary depending on light conditions etc.
74HC family can run on 2-6VDC power,
74HCT requires 4.5-5.5VDC,

I think I would try 4000B series chip like 4093B etc.
CD40106 only works down to 3 volts, which is probably above the trigger threshold of the circuit.
In order for this circuit to "jump" (momentary motor pulsing), the solar cell current output is necessarily too low to supply continuous motor current. Therefore, the Schmitt triggers only have to work up to the trigger threshold of the circuit, which is probably less than 3 volts. The open circuit output voltage of the solar cell is irrelevant, so the fact that 74HC14 only works up to 6V is also irrelevant.
 

panic mode

Well-Known Member
solar cell voltage is very relevant. the point was that circuit operating voltage and supply voltage need to match. i have two solar cells that are 12V and 24V for example. the 24V is too big for bug-like robot but the 12V would be ok.
 
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