# CEM3340 Simple VCO - PWM help **URL:** \ **Category:** DIY STUFF\ **Created:** [June 15, 2021, 1:09pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-simple-vco-pwm-help/3654 "2021-06-15T13:09:00Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![miyagi](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/miyagi/32/15345_2.png) [@miyagi](https://lookmumnocomputer.discourse.group/u/miyagi)\ **Post date:** [June 15, 2021, 1:09pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-simple-vco-pwm-help/3654/1 "2021-06-15T13:09:01Z")
> **[STUFF - LOOK MUM NO COMPUTER](https://www.lookmumnocomputer.com/projects#/cem-3340-diy-simple)** > > Click on the images for more info on the projects, this is a work in progress and will build on the pages as i go. I built this a couple days ago (the square out version with PWM but not PWM CV), and triangle and saw sound awesome. Square sounds ok, but the pwm pot on it has very limited range. I only get a good square tone in the middle range of the b100k . if i turn it up or down more than 20 percent it starts cutting out. Is this to be expected or what could be wrong? I am pretty new to all this so be patient please. 🙂 ---
**Author:** ![fredrik](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/fredrik/32/682_2.png) [@fredrik](https://lookmumnocomputer.discourse.group/u/fredrik)\ **Post date:** [June 15, 2021, 1:18pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-simple-vco-pwm-help/3654/2 "2021-06-15T13:18:46Z")
It’s expected. The PWM CV pot range extends outside the usable PWM input range: > [@Simple 1v/oct oscillator issue](https://lookmumnocomputer.discourse.group/t/simple-1v-oct-oscillator-issue/999/5): > > The square wave is generated by running the sawtooth through an internal voltage divider and comparing it to the control voltage, which gives you an effective range of just over 0 V to just under 4 V (supply/3). The circuit still works, it’s just that you cannot hear a signal that’s 100% on or 100% off smiley (this is common in DIY circuits since the alternative is trimpots everywhere, and you need to be able to tweak your CVs anyway so people usually don’t care that much about what happens a… ---
**Author:** ![analogoutput](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/analogoutput/32/29663_2.png) [@analogoutput](https://lookmumnocomputer.discourse.group/u/analogoutput)\ **Post date:** [June 15, 2021, 1:42pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-simple-vco-pwm-help/3654/3 "2021-06-15T13:42:50Z")
You can use a multimeter to check what voltage range is usable (if it’s really ±20% from the center then presumably it’s about 3.6 V to 8.4 V) and then use fixed resistors in series with the pot to get that range from the pot. If I’m doing the math right then a 75k resistor at each end of the pot will give you that voltage range. (Or you could use 100k trim pots for adjustable voltage limits.) ---
**Author:** ![fredrik](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/fredrik/32/682_2.png) [@fredrik](https://lookmumnocomputer.discourse.group/u/fredrik)\ **Post date:** [June 15, 2021, 2:07pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-simple-vco-pwm-help/3654/4 "2021-06-15T14:07:05Z")
> [@analogoutput](#): > > (if it’s really ±20% from the center then presumably it’s about 3.6 V to 8.4 V) The CV pot is connected between 0 and +12 V with the wiper connected to a voltage follower (via an extra 100k that doesn’t really do anything until you add a CV input), and the input expects 0 to 4 V, so if you only get a useful signal in the middle of the range something’s weird. I’d double-check the wiring, then add a trimmer on the +12 V side (see drawings in the linked thread). ---
**Author:** ![miyagi](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/miyagi/32/15345_2.png) [@miyagi](https://lookmumnocomputer.discourse.group/u/miyagi)\ **Post date:** [June 15, 2021, 2:18pm UTC](https://lookmumnocomputer.discourse.group/t/cem3340-simple-vco-pwm-help/3654/5 "2021-06-15T14:18:27Z")
ill check the voltages when i get home from work. and update after that.