Doepfer MCV4

I use this unit to trigger sounds on my Roland SH101. The 101 only responds with certain octaves so I'm glad the unit has an LED that lights up when midi notes are sent to it so If I get no response from the synth I can quickly figure out where I've gone wrong.
Set up is very easy. You can change what midi channel the unit responds to by holding down the Gate/Learn button until the LED flashes, send it any midi note on the channel you want it to listen to, the flashing stops and now it only listens to that channel.
It can also do other tricks such as transposing notes but I never had any need for these functions, if you do start experimenting with it settings and get confused, it's very easy to return it to factory settings.