555 makes rails oscillate

[Megamox]

Im using a 555 in astable mode with equal mark/space ratio and connecting to transmitter, but it seems even when the output of the 555 is not connected, the tone is still transmitter, i guess its oscillating the rails which the tank circuit on the transmitter is connected on. I just add a large capacitor across the rails to stop this right?

 

[Styx]

yup you should always have large tank caps near the source of the supply (ie the 5v) and also local 100uF (ceramic or better) right next to the IC across teh supply pins to couple the high-freq noise to GND

 

[Russlk]

You cannot get equal mark-space ratio from a 555. If you try, it will draw mucho current from the rail and thereby screw up the power. Run the 555 at 10% duty cycle and double the frequency, then use a divide by 2 to get 50% duty cycle.

 

[TheOne]

You can get 50% duty cycle without any problems by inserting "steering" diodes to control charge/discharge paths separately. R2/3 can be adjusted to get 50%

Alternatively a DC control voltage on pin 5 can be used to adjust the duty cycle.

Another way is to use a Jfet to replace one timing resistor. Frequency set by R3,C2 combination.