RE : RadioRon
thanks l have done the modifications in the attached circuit and am looking simulation software for the circuit. other views are welcome
I use the free simulation software called LTspiceIV from Linear Technology.
The modifications are correct, you now have a topology that makes sense. I recommend that you follow Audioguru's recommendations on improving the transmitter section.
Your switch, Q1, is generally the right idea but not optimal. One problem that will occur is that the voltage divider of R7 and R8 will reduce the voltage presented to the base of Q1 to a very low value and so the emitter of Q1 will always be too low a voltage. You will need to improve the design of this switch. The most common successful configuration for a bipolar transistor switch like this is to use a PNP transistor (eg. 2N3906 or any other) with emitter as input and collector as output. Unfortunately, this PNP arrangement will require that your control signal be "active low" instead of active high as it is now. In other words, the base of the PNP switch must be pulled low through a resistor to turn the switch on, not high through a resistor as you have it now. However, you can easily reverse the polarity of your control signal by reversing the direction of D1. For the suggested configuration, you would also need to move R8, from its current location, to making a connection from +9V to the junction of D1 and C3. I suggest that R8 would become 100K ohms and R7 should become 18K ohms. C3 may be OK at 47 uF, but you may find this too small, perhaps 100uF would provide a longer "hang time" for the transmit function.