Replies to Original Minipulse Project Details

[Qiaozhi]

I'm now working on a Minipulse II version with some good results.

The transmit rate has been increased to 1000pps with a pulse width of 100us. Quiescent current consumption is 86mA.

Although I read somewhere that this detector was sampling with a 20us delay, it was actually measured as 58us. Although, with the components fitted, the sample time should have been 30us. I have made this adjustable, and managed to sample at 22us using the original coil (543uH). The MC14538BCP (U12a) has a minimum possible delay of 10us. This is achievable with the delay adjustment, as long as you have a faster coil.

A self-adjusting threshold (SAT) has been added, which is a great improvement. When you adjust the Reject control, it no longer needs to be retuned. The SAT is quite slow, so there is no loss of depth.

A small change was made to the final integrator stage that has greatly improved stability and the rejection of external noise.

The pre-amp has remained as a UA709 (I don't want to overdo things with the original PCB) but will be replaced by an NE5534 in the Mark II design.

Mk II PCB layout coming soon, together with wiring to the external controls, headphone / charger socket, etc.
Please be patient, as I'm very busy and trying to do 15 things at once.

[sinclairuser]

very interested in a board would definately buy a kit.

[Tepco]

Few issues with 4538 connection on mini PCB. Pins 8,1,15 are Vss, timing capacitor goes from pin 2 or 14 toVss, but on original PCB pins 1 and 15 are not explicitly connected to Vss (-5V in this case). Pins are internally connected inside chip itself (thru some very low resistance path), will work connected like this, but data sheet say pins 1,8,15 must be grounded externally, usually they are in most designs. Also timing capacitor marked C24 on PCB, from pin14 U12b is missing on schematic.


Just another rainy day to finish this, I already retraced PCB in Protel from pictures, only for component placement, now checking connections.

[Tepco]

Progress so far, relatively similar to original, just small cosmetic changes.

[Qiaozhi]

Few issues with 4538 connection on mini PCB. Pins 8,1,15 are Vss

The MC14538BCP has pins 1, 8 and 15 connect to -5V.
So this is correct.

timing capacitor goes from pin 2 or 14 toVss, but on original PCB pins 1 and 15 are not explicitly connected to Vss (-5V in this case).

I also noticed this. The original PCB designer assumed pins 1, 8 and 15 are internally connected in the chip, which is true, but it is good practice to explicitly connect them to -5V.

Pins are internally connected inside chip itself (thru some very low resistance path), will work connected like this, but data sheet say pins 1,8,15 must be grounded externally, usually they are in most designs.

As above.

Also timing capacitor marked C24 on PCB, from pin14 U12b is missing on schematic.

Damn. It's on my hand-drawn schematic, but got missed off when I drew the schematic. Thanks for spotting that.

Just another rainy day to finish this, I already retraced PCB in Protel from pictures, only for component placement, now checking connections.

If you find any other issues, please let me know.

I've updated the schematic with the missing cap, and added the improvements mentioned in a previous post.

[Qiaozhi]

Please see the final schematic for this design (with all errors corrected) in the sticky post at the top of this section ->
Original Minipulse Project Details