The Doepfer A-151 is a Quad Sequential Switch. Module A-151 (Quad Sequential Switch) is like an electronic four-position rotary switch. It includes trigger and reset inputs, four in / outputs, and a common out / input. Each time a pulse is received at the trigger input socket, the common out / input is connected to the next in/output. After the fourth in/output, the next trigger makes it step back to the first again, and so on. A positive pulse at the reset input switches the out / input immediately back to the first in/output. Voltages in the range -8V...+8V at the O/I resp. I/O sockets can be processed by the module. Four LEDs indicate the active in/output (ie. the one that is connected to the out / input at any particular time). From spring 2005 a revised version of the module is available. With an additional switch the number of steps can be set to 2, 3 or 4 (thanks to Peter Grenader for this idea). Moreover the revised module allows to process voltages in the full A-100 voltage range (i.e. -12V...+12V). The limitation to -8V...+8V is no longer valid. While stocks last the user can choose between version 1 and 2. Version 1 will be no longer manufactured in the future.

The main features of the Doepfer A-151 include:

• Width: 4 HP / 20.0 mm 
• Depth: 35 mm (measured from the rear side of the front panel) 
• Current: 40 mA 
• Trigger and reset inputs
• Four in / outputs
• A common out / input
• Four LEDs indicate the active in/output