The procedure for determining the frequency-weighted RMS values of vibration accelerations in the case of petrol chainsaws proposes equal time shares of three operation modes of the chainsaw. In order to assess the actual operator’s vibration exposure, it is necessary to precisely determine the above-mentioned shares. In real-life conditions operation of chainsaws is characterized by frequent and significant rotational speed changes depending on the operation mode. Therefore, a prototype of dedicated device for measuring the rotational speed of a chain sprocket was developed. The article presents development requirements, encountered problems and their solutions, the prototype itself and functional test results.