555 in astable mode

Hi all,

I am trying to create a astable circuit to generate a 56khz carrier frequency for infrared but first i want to build the astable circuit using a 555 timer i have used the information online using this website here

http://www.ohmslawcalculator.com/555-astable-calculator

the values i have used are C - 0.1nf, R1 18k, R2 120k to generate 55.930 which is the closest i could get to 56khz with a duty cycle of 53.49.

it is to my belief that a lower duty cycle is better? (as i understand this means i can use more current)

i will be using a tsop 4856 to receive and a tsal6100 to send i have check the power supply on the oscillator using
a arduino 5v and a dedicated power supply.

My problem i am having is on my oscilloscope i am getting 43.72khz on the frequency with these awful surges at each rise in peak, i have attached a picture below if anyone can take a look it would be appreciated.

P.S i am also using 1% tolerance resistors and ceramic capacitors with a NE555N timer.

Thanks in advance, Chris.

There is no attachment!

1% resistors but what tolerance on the capacitor.

Some ceramic capacitors are 20% and not very temperature stable.

If you find you need 50% duty cycle then consider the CD4047. The oscillator operates at twice the output frequency because of the internal flip-flop.

Common mistake - you didn't link an image, you linked a website!

A "555" covers a variety of chips, one should generally be using the CMOS version.

The "surges" are generated in your wiring.

Arctic_Eddie:
Some ceramic capacitors are 20% and not very temperature stable.

Yes, use NPO caps, not bypass ceramics.

I have tried three different types of capacitors with different tolerances neither have a difference which is strange. also this doesnt explain why i get 43.96khz instead of my 55.930khz.

Try another 555 from a different vendor as the switching point may not be as specified.

the values you can used are C - 1nF, R1 7.5k, R2 9.1k to generate 56.148 kHz

too low capacitance had problem to stable.

Why not use a 14.31818 crystal oscillator and divide by 256? That will get you an extremely stable 55,930.4 Hz. You can use a 74HC4060 for a one chip solution since it has oscillator pins.

Shandy:
it is to my belief that a lower duty cycle is better? (as i understand this means i can use more current)

Consider adding a diode as described here.

aarg:
Why not use a 14.31818 crystal oscillator and divide by 256? That will get you an extremely stable 55,930.4 Hz. You can use a 74HC4060 for a one chip solution since it has oscillator pins.

Full marks for that!

Perhaps that is why 56 kHz is specified in the first place!