From ericgibbs
Do you have a series resistor from 555 pin 3 to the base of the BC549.??
No, I have just wired the o/p direct to the base. Should I need a resistor there?
From dougy83
The LED frequency will be the same as the 555 output frequency. Use a CRO to view the waveform if you don't believe it.
Ermm, that's how I know the frequency is different - I am measuring it with a CRO
Just to clarify, I am seeing a difference in the frequency between the following 2 readings.
a) the
unconnected output from the 555 (i.e. a wire taken directly from pin 3 and NOT connected to anything else
b) the collector of the transistor (which is also the negative lead of the LED) when the 555 o/p is
connected to the transistor base.
It looks as though the resistor, LED, transistor 'group' somehow affect the input signal frequency, but how/why I have no idea.
The point you make about switching the carrier is something I had missed
So, it looks like I will need to do 2 things to generate a suitable signal
1) generate pulses at the required frequency, say 37KHz
2) use another circuit to break switch this on for 600us then off for 600us
Hmmm, this simple(?) project is getting more and more interesting by the minute...
BTW, how do you guys insert bits of other peoples appends so that they are referenced correctly? I am cutting and pasting bits of your appends, then highlighting and selecting the 'Wrap quote tags around' icon, but there must be a better way that also gives the quoter details, and the link?