Tech Question - TX switches

[geoscash1]

I have been working on a circuit and have researched many of the existing stuff that floats around but have a question regarding switches in the TX section.

Example in pic below in the TX portion, this particular one is from the hammerhead although I see many using this approach.

*What is the purpose of using 2 switches after the oscillator output and before the mosfet?

Can these be eliminated and directly pulse the mosfet from the oscillator and if not why?

TX section switches circled


Thanks in advance for your replies
Geo

[mickstv]

The 555 timer is connected between 12v and -5v so Q1 is used as a level switcher, the output of Q1 will be between 12v and 0v. Q2 is just a buffer, which could be changed with a mosfet driver. A better approach would be to use a opto driver, then you can get rid of Q1, Q2 and the resistors involved.

[geoscash1]

The 555 timer is connected between 12v and -5v so Q1 is used as a level switcher, the output of Q1 will be between 12v and 0v. Q2 is just a buffer, which could be changed with a mosfet driver. A better approach would be to use a opto driver, then you can get rid of Q1, Q2 and the resistors involved.

Hey thanks mickstv.. I think I have some mcp1407's which are mosfet drivers but will either of the below work as well?

Optocoupler driver, would this one be something that may work - MOC3023
http://html.alldatasheet.com/html-pd...1/MOC3023.html

or MOC3063
http://pdf1.alldatasheet.com/datashe...A/MOC3063.html

[mickstv]

Hi Geo, the MC1407 will work in place of Q2 but you'll still need Q1 as the level shifter. This is the easiest method if your modifying a hammerhead detector.

[Carl-NC]

Mick is right, Q1 is used as a level shifter. Q3 could have been connected directly to Q1-collector but then it would have a resistive pulldown. To get a faster turn-off I added Q2 as an active pulldown.

There are better ways to do this, but I intentionally kept it simple so that people wouldn't have to find esoteric parts just to build the circuit. An opto driver would work, just watch out for slow response times.

[Ferric Toes]

Mosfet Drive etc.

Below is a versatile pulse generator for TX, delay and samples, plus synchrous supply + rail and -ve rail for the RX. The pulse generator is between the +5V and -5V rails so that the drive for 4066 type Cmos gates is correct (SA1 and SA2)
This shows the use of a complementary emitter follower to give low impedance on/off drive for the Mosfet. Resistor in the gate is 10R to prevent spurious ringing in gate drive circuit from track inductances etc., but with good layout probably not necessary.
The TX Inhibit is for a switch to turn off the output stage, but leave everything else running. Handy when the TX mean current is 5A and you are digging a 3ft hole. The two 'and' gates giving SA1 and SA2 give a half width sample pulse. Full width ones appear on pins 6 and 8 respectively. Additional delays and samples can be picked off the 4017 counter if required. You can also run two counters in cascade to give up to 17 counter outputs for greater flexibility. A pot can be wired to PL1 for 'one' control operation and the preset used to set the minimum delay required.

Ignore second image as it has an error. The B version as displayed is correct.

Eric.