Experience with 555 Timer Chips

[Bulldozer2003]

I am trying to get a 555 timer to give me a 14Hz signal. I followed this website using only the left 555 circuit (I did check the pinouts since this site used a 556 chip) and found that it matched the schematic in the manufacturer data sheet for my timer chips (Harris HA17555). I rebuilt the circuit twice already and am 100% sure it is correct, am I overlooking something? I tried two chips, could both be fried?

I don't need to do pulse width modulation. All I want is a voltage spike 14 times a second that lasts long enough to drive a mosfet driving an array of LEDs. I tested the above circuit with a white LED. Is it possible not enough current is being sent to the LED? Should I try a higher duty cycle?

 

[BEL6772]

I know you said you rebuilt the circuit, but are you sure of the polariy on your test LED?

Do you have a scope available?

You did connect by pin name, not pin number on the schematic you were using, right?

I did a google for "astable multi-vibrator + 555" and found several labs that showed wiring it up with a 555 instead of the 556. Also, this site showed up. It is the best one I saw for describing what I think you're trying to do.

Hope it helps

 

[Wizkid]

If you have a copy of multisim you might want to give that a try first... then you will know if your circuit works before you try to build it.

 

[bobsmith1492]

Try one of these:

http://ourworld.compuserve.com...es/Bill_Bowden/555.htm

Remember, simpler is better, especially as a 555 timer circuit is as simple as it gets.

Edit: Try the astable configuration.

 

[Bulldozer2003]

Thanks, I'll try these suggestions.

 

[kedvale]

One easy way to test you can do to confirm that your LED circuit is correct is to disconnect the output of the 555 circuit, and directly connect the gate of the drive FET to VCC and GND (not at the same time, of course!), and make sure the LEDs turn on/off. If the LEDs behave as expected, you have isolated the problem to your 555 circuit. If the LEDs do not light, then you need to figure out your LED circuit first (as a previous poster mentioned, check the polarity on the LEDs...also, how did you calculate the series resistance? Make sure to look up the forward voltage of the LEDs and the minimum current required on their spec sheets and use that info to choose the proper series R). Hope this helps.

 

[Bulldozer2003]

Excellent I got it working with the astable mode but I see that a 50% duty cycle is the best you can get. Is there an arrangement where I can get less then a 50% duty cycle, other than a CMOS inverter on the output?

 

[Bulldozer2003]

No body knows of another device for creating simple oscialltions? Although I admit the 555 is pretty simple as it it.

 

[FrankSchwab]

Try this link, search for "Extended duty cycle astable". Shows a circuit that allows creating duty cycles both above and below 50%.

Unfortunately, you need to vary two components simultaneously to do what you want - You'd need to change R1 and R2 in tandem, with one rising while the other is falling. I'm guessing that by using a linear pot, and using the wiper as the connection to pin 7, and the two end connections of the pot as the connection to pin 4 / pin 6, you'll be golden.

As an example, using a 10 uF capacitor and a 10K ohm pot, you'll end up with a frequency of
f = 1.44 / (R * C)
f = 14.40 hz.

When the pot is set near one end (assume R1 = 100 ohm, R2 = 9900 ohms)
High Time = 0.69 * R1 * C = 0.690 ms
Low Time = 0.69 * R2 * C = 68 ms
Duty Cycle = 9%

When the pot is set near the other end, (assume R1 = 9900 ohm, R2 = 100 ohm)
High Time = 68 ms
Low Time = 0.69 ms
Duty Cycle = 91%

Good luck,

/frank

 

[JimPhelpsMI]

Hi, 555 duty cycle is set by the ratio of the resistors connected from + (pin 8) to pin 2 (R1) and pin 2 to pin 6 (R2). Charge time is R1 + R2 and the cap and discharge is R2 and the cap. Small value of one and large value Of the other will give spikes if you want. A diode with anode to pin 2 and cathod to pin 6 will allow 50% duty cycle using same values for both resistors. Hope I got the pin numbers correct. It's been a very long time since I used a 555. Jim

Edit: 555 will sink 200 ma. to ground and R1 will determine the High current. It has no problem running one led. Jim

 

[Bulldozer2003]

What would people reccomend to create an exact frequency of 14.035 +/- .003 Hz? Should I just try to program some style of PIC chip?

 

[bobsmith1492]

Use a crystal and dividers? Wait.... is that just 14 Hz, no megas or anything? Nevermind....

You could use a crystal and some counters (4017? 74LS121?) cascaded to divide it down from its 3 MHz or whatever to 14. I have no idea how to get it that accurate
Sure am helpful here..... sorry.

 

[JonB]

You would have to use a higher frequency and divide it down. Temperature stability would be the biggest problem, even with crystals.

use a counter. The reset output could be the 14.035 hz signal.

make it count to 1000 with a 14,035 Hz input and you'd be pretty close.
make it count to 1,000,000 with a 14.035 MHz input and you'd be near perfect.

btw, does that specific frequency cause odd behavior in humans? Epilepsy? Nakedness? Nausea?

Take an output from the PCI bus on your computer (33/66 Mhz) and set a counter (or a series of counters) to 4,702,529. The reset line would give you a pulse 14.03 times per second.