• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Circuit needed

Status
Not open for further replies.

crunch

New Member
I have played around with a circuit that flashes 8-10 led's randomly at a second or 2 on and 10-15 seconds off using 555's. What I'm wondering is... is there another single chip that could be used to do about the same thing that the 8-10 555's are doing and make it simpler. Would there be a simple micro processor that would do the same thing that could be programable? Any info you could provide would be greatly appreciated since I'm not real good at designing circuits. I'm pretty much just beginning.

Thanks,

Mike
 

bogdanfirst

New Member
well it is not random....
all the leds have a fixed frequency with wich they flash, but the frequencies are different so it gives you the impression that they flash random
one "simple" thing it to connect them to a computer and use a software to control them, but you say that you are a beginner.
i dont know you can use a smaller number of ics.
you couls use an EPROM and program it so that the sequence seems random.
i think that the 555 version is the simplest, but you have to build the circuit 8-10 times, each one for a led.
the 556 has 2 555 timers inside so you can reduce the number of ic's to half.
i will think of something else.
 

Gene

New Member
I'm afraid that if you want a design that is simpler than yours, you will probably have to show us yours.
 

bogdanfirst

New Member
thats true.
but from what i understand you just have 10 leds connected to 10 555 oscilators, and thetre frequency is different so you have the impresion that the 10 leds flesh randomly.
 

bogdanfirst

New Member
hey i think i have an idea....
use a 555 to generate pulses of a highter frequency and another 555 to generate a smaller frequncy. use a counter wich gets the signal from the first onew only when the ouput of the second 555 is high and for each outhput of the counter connect a FF and drive the led with it. this way you will have a little bit smaller number of ICs
but i dont think it will work.....
 

crunch

New Member
Thanks all for your help. I thought it was about the simplest circuit. Mainly I am trying to use fewer components. I'm not sure, but it looks like the 558 will help out a lot. I'm just not sure yet if each section can be set so the led will be on for a second and off for about 8 seconds and then the other 3 sections off just a little bit from there so it will look random. Then it would just take 3 chips etc.
 

Gene

New Member
Cycling (on-off) times of the 555 are adjustable. Normally, we attempt to use a 50% duty cycle which approaches a true square wave. In this configuration the pulse 'on' time is the same as the 'off' time. However, if you are using a standard R-C (resistive-capacitive) circuit to set the timing pulses, you can change their values to a wide array of duty cycles. For the formulas, do a web search for 555 calculators - there are quite a few available that will do the math for you.
 

bogdanfirst

New Member
but how do you build a 555 with a 50% duty cicle?
i think it is a bit more complicated. probalby some diodes are needed
 

arcom

New Member
Hi crunch.

Instead of using 8-10 555's (or 5 556's), you can build a circuit with just one IC by using a microcontroller. If you use for eg. PIC16F84 you can connect up to 13 LEDs and create a truly random flashing sequence.
 

bogdanfirst

New Member
well, with the normal oscilator you cannot get a square wawe beacause the charging is done trough one resistor and the discharging is done trough that resistor and another one. so you can do, as i see from your values put a resistor with a value much bigger than the other one and you will get a nearly square wave. but i think that for 50.9% you cannt notice that without mesuring, because the frequency is qite high.(>1Hz)
 

kinjalgp

Active Member
bogdanfirst said:
but how do you build a 555 with a 50% duty cicle?
i think it is a bit more complicated. probalby some diodes are needed
Not at all complicated, only a single diode in between pin 7 and pin 6 in astable configuration.
 

bogdanfirst

New Member
well i think i know how. but a little schematic from you will do.
p.s. you promised me that high gain audio amp....
 

kinjalgp

Active Member
Ya, sure why not. i haven't forgotten that. But, please wait, I got to search my old little circuit book, in which I used to draw my schematics in school days when I didn't had a computer. :)
 

crunch

New Member
arcom said:
Hi crunch.

Instead of using 8-10 555's (or 5 556's), you can build a circuit with just one IC by using a microcontroller. If you use for eg. PIC16F84 you can connect up to 13 LEDs and create a truly random flashing sequence.
This sounds more like what I'm looking for if it is fairly simple and uses less components. I was wanting to use the 555's only because I was a little familiar with them. The more random the better. I could use something like that in other things also. The next problem is how would I program them. I don't have a programmer. If you think this is the way to go and you are familiar with it please send me any ideas you might have and links to places with more information.

Thanks,

Mike :D
 

kinjalgp

Active Member
bogdanfirst said:
well i think i know how. but a little schematic from you will do.
p.s. you promised me that high gain audio amp....
Ok here is how you generate 50% or less duty cycle from 555. Here R1= R2.
 

Attachments

crunch

New Member
kinjalgp said:
bogdanfirst said:
well i think i know how. but a little schematic from you will do.
p.s. you promised me that high gain audio amp....
Ok here is how you generate 50% duty cycle from 555. Here R1= R2.
What would the values be to have it 1 second on and say 8 seconds off? Also, Is the diode necessary?
 

kinjalgp

Active Member
The diode is necessary only if you want on time = off time i.e. 50% duty cycle or less (<50%). Otherwise it works without it also.

For caluclations of above circuit:
T_on = 0.693 * R1 * C
T_off = 0.693 * R2 * C

and for circuit wihtout diode:
T_on = 0.693 * (R1+R2) * C
T_off = 0.693 * R2 * C

For your requirement, duty cycle is less than 50% (on time is less than off time). Therefore you have to use the above circuit with diode. The other will not work.

Assume any value of capacitor say 100uF for your case and then calculate values of R1 and R2.

1 second = 0.693 * R1 * 100E-6
Therefore, R1 = 14.43 k

Now,
8 seconds = 0.693 * R2 * 100E-6
Therefore R2 = 115.44 k

Enjoy! :)
 

Scubasteve

New Member
hi Mike,

Believe me, you'd get a lot more satisfaction by doing this the more discrete way rather then resorting to a micro. Microcontrollers can be tedious to learn and hard to get the parts for it to program and the support components (crystal, caps).

But if you feel that you have the time, money, and initiative to it, go for it!

If I were you, I would take the suggestion about using a low and high frequency oscillators with two binary counters. This would give you several frequencys to play with without the cost of making several oscillators.

Steve
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top