ZoneMikel's Blog

I hope to be able to create a 40kHz signal to drive my transmitter for my range finding sonar. It has to be very close to 40kHz and tunable would be better to get the max output.

From the very simple circuit on this page the frequency is given by
Frequency = 1.44/(C1*(R1+2R2))
Where R1 and R2 are in megaohms and C1 is in microfarads.

So, f=1.44/x; x=0.000036. so if we let c1 = 1uF then R1+2R2=36uf. If we let R2= 1M then R1=36uf-2 which makes no sense. If we let R1 =1M the equation becomes R2 = (36uf-1)/2 and R2 becomes -.49 … so we need a superconductor or another value capacitor.

(after a night and half a day)

If (R1+2R2)=10 then C1 is 3.6uf. But that “r1 and r2 in M and C1 in u” is really confusing… I really dont like that formula. If you go to wikipedia they pretty much have the same formula.

If i use R1+2R2=10M then C comes to be 3.6pF. The smd caps I have that are close to that value are 1, 2.2, 3.3, 4.1 and 8pF. So with the 8 the resistors must be about 4.5M. I dont know why but I would rather have smaller value resistors, so I’ll go with the 8pF cap. With that cap I could have a 2.5M and 1M resistor for R1 and R2. The resistors I have on hand in the M’s are 1,2.2,3.3 and 4.7M. If I use a 2.2M for R1 that gives R2 as 1.15M when solving for R2 in (R1+2R2)=4.5M. For the 1.15M I’ll use a 1M and a 150K in series to give 1,150,000 ohms. Besides the tolerances this should give me something really close to 40kHz.

40kHz: C=8pF, R1=2.2M, R2=1.15M
According to those values and the forumla for frequecny above I get 40,075Hz.

Constructing the circuit I get a weird duty cycle that is more like 80% and the frequency is all wrong … I’ll have to investgate more and test in multisim.

(a little later)

When I tested it in multisim the results looked very similar to what I saw on my scope. A frequency of about 30kHz and a duty cycle of like 80%. I found a online calculator and used my values, it said my duty cycle was 74%. So my calculations all give me 40kHz but I’m not getting that.

A quick search on the web for “555 timer 40kHz” yeilded this

And a few links down I found this … which pretty much solves all my problems. Now its just a matter of building it and testing it with a scope.

Now I just need to find some pots … And i’ll also test to see if we get a big signal boost by adding a inductor. From my tests with multisim a inductor in parallel with the Txer will give voltages in excess of 20v from the 5v source.

PART II

Well after a few days (and getting my soic 555’s) I decided to test this. I created a circuit to test it in multsim, then ultiboard then I etched it using the process described in my other post

This circuit worked great ! By counting the us/div etc on my scope i found it goes from 29,851Hz to 41667Hz which works fine for this. I also feel pretty confident that i could replace the pot with a 11k resistor and not worry about it, but for tweaking the pot is nice. I set it to 40kHz by using the formula x*(s/div)=To where To is the period and the (s/div) is whatever you set your scope to.

Then I hooked it up to some of the tx/rx pairs i have it did work. There seems to be a lot of bleedthrough from the tx to the rx via the circuit but if i connect my scope to a separate rx and hold it close it will get a good .75v when it is close. As is i think this will work for ranges of about 8inches. Something has to be done about removing the ac from the tx/rx pair, there is already a cap in there as they had in this picture but it either is not working or does not work (lol).

Oh and that 330uH inductor does not seem to do anything, it just wipes out the signal and gets really hot. But all this is for another post when i make the ultrasonic tx/rx boards. For now the timer is established that it works great.

So the whole timer circuit is connected to one pin on the mcu, this one pin goes to the reset pin on the 555 timer. It should have a pull down resistor so it is low when its not high (lol). When you send a high to the reset the 555 will send out a ping ( or a few depending on how long you turn it on). Then we just wait for the reply via the us rx (gt .2v should work).