Answer
As the one answer said as long as it is on your own time and your own equipment there is nothing they can say, unless you are using their ideas or writing software that competes or works in their industry. The later would be a conflict of interest and you can be sued for that. If you don't do it while at work. You don't use any company equipment or software. And you don't use any company ideas or resources then you are free to do it. If you have a good relationship with you boss it doesn't hurt to mention it to them, just in case. Note: if you did sign something as someone else suggested that is not enforcable, at least in Canada, if you can prove you didn't not use company equipment, etc.