MPP Beach Step 3 confusion (of mine ;))

[Blobbob]

Hi folks,

The following is just a theoretical consideration of the schematic, I don't have a MPP nor a minipulse yet

I am a little bit confused about the settings in 'step 3' for the transmit oscillator.
We should monitor the the TX oscillator output on TP2 to set up the TX pulse to 400us width an 8 ms period.
But the signal on TP2 is connected to a n-ch mosfet. Wouldn't that mean that the signal is inverted?
So if I set on TP2 a pulse for 400?s (on-time) in a periode of 8ms would become after the mosfet a 7,6 ms pulse?


Also could it be .. that R5 and R14 are swapped in the description of step3?

[Qiaozhi]

Hi folks,

The following is just a theoretical consideration of the schematic, I don't have a MPP nor a minipulse yet

I am a little bit confused about the settings in 'step 3' for the transmit oscillator.
We should monitor the the TX oscillator output on TP2 to set up the TX pulse to 400us width an 8 ms period.
But the signal on TP2 is connected to a n-ch mosfet. Wouldn't that mean that the signal is inverted?
So if I set on TP2 a pulse for 400?s (on-time) in a periode of 8ms would become after the mosfet a 7,6 ms pulse?


Also could it be .. that R5 and R14 are swapped in the description of step3?

As it says on page 4 of the Build Document: "The positive (+ve terminal) of the battery pack is treated as ground (0V) in this design. All voltages and currents are referenced to this point.".
This is clearly shown in Fig.5, where the pulses are negative with respect to 0V.
As you correctly point out, the 555 output will appear to be upside down when referenced to the 0V line. However, the 555 and the MOSFET are both referenced to the -VB line, so (as far as they're concerned) it's the right way up.
The power supply arrangement is similar to the Hammerhead, and it can be somewhat confusing if you're not used to it.

[Blobbob]

Arrg, yep NE555s positive output is ground ... missed that one more time.
Thx

But what about the swapped width and period potentiometers, also a result of the positive terminal equals to ground confusion?^^
Had a quick sketch on spice, and R5 influences the period time instead of the pulse width

[Qiaozhi]

Arrg, yep NE555s positive output is ground ... missed that one more time.
Thx

But what about the swapped width and period potentiometers, also a result of the positive terminal equals to ground confusion?^^
Had a quick sketch on spice, and R5 influences the period time instead of the pulse width

Look at Fig.4.
The 555 timer is referenced to -VB. Bearing that in mind, and assuming you have the ground of the scope connected to -VB; when the 555 output is high, R5 (100k) is the timing resistor for the pulse width since D1 will be forward-biased. When the 555 output is low, R14 (1M) is the timing resistor for the period since D6 will be forward-biased. C3 charges via R5, and discharges via R14. Also note that R5 is 100k and R14 is 1M. This should be a clue as to which resistor is associated with the pulse width, and which is associated with the period.

I think you're getting confused with the +ve battery being used as 0V. Trust me ... the circuit works as advertised.

Try changing your Spice circuit with 0V connected to pin 1, and +5v to pin 8.
Also, look at Fig.5. You can clearly see that the TX pulse goes negative when the measurement is referenced to the positive battery terminal.

In this design, the 555 timer uses the output pin to control the charge and discharge of C3. The discharge pin (7) is not connected.

[Blobbob]

thank you for the fast and detailed answer, that's fun to work with ;D

[nonkapo]

When i start building my Minipulse plus i found a very handy Youtube video which I'll post here, so everyone can see the step by step setup and the oscilloscope pics as in the Geotech manual . I don't know who's the guy responsible for it but he deserve a lot of beer(gues it might be some of you guys;-). Hope also he or anyone else do not mind i post the link to that video here.
https://www.youtube.com/watch?v=OyS8sfv0XJ0