Hi,
I am working on this small project kit for a class in school. It is the velleman mk120 ir light barrier kit. Basically I am going to enhance it a little bit using my de1 quartus board connecting the transmitter to the rs232 port. But first things first, I am trying to figure out what capacitors I will need to increase the frequency from 6KHz to about 30KHz on the transmitter. I tried putting the schematic onto LTSpice but it seems like the smallest capacitor I can put on the circuit and get it to simulate correctly is about .01uF when the default on the original circuit is about .0015uF. I pretty much copied the original schematic, so I'm not sure whether there is an internal discrepancy regarding the components in the ltspice schematic or something else.

Here's the schematic from spice: http://share.pdfonline.com/4bfb0e2b74bb4aa9b8d239119dd3e4e7/projectschematic.htm
Let me know if it is difficult to see because when I uploaded it, some of the lines appeared somewhat faint.

The original schematic should be attached.

Thank you

 

Yes, the linked drawing is poor quality, but it doesn't matter. The schematic on this page is good.

Increase frequency 5x means decrease time constant 5x.
You already have a demand for the transistors to supply a gain of 100 so I'd say reduce the 100k resistors by 5x.
That makes the capacitors charge 5 times as fast and lowers the required gain by a factor of 5.

 

Ok thank you. This is probably a simple question but if I am powering the transmitter with a 9v battery, I would also need an AC signal in order to produce the proper frequency, correct?

 

Nevermind, figured it out.