Beat Freq

From a beat frequency indicator schematic that has floated round the web for years: fed two signals, a ring of four LEDs turns clockwise or anticlockwise depending on which has the higher frequency, faster the further apart they are; at the same frequency a single LED stays lit. The original's 74-series logic is redone in CMOS, with a gate output for each LED and the spare outputs of one chip feeding four pots to make a CV sequence; a discrete XNOR circuit gives a pseudo ring-modulator output from the two inputs. The CV returns to 0 between steps, each step's length varying with the inputs; the gates come in spurts and the ring jerks around, several confident revolutions, then a stutter or a shimmer on one spot, unpredictable at audio and clock rates alike but always putting out useful signals. The LEDs shine through the bare board of the panel's X.

Key features

• IN1 and IN2 in; four gate outputs, CV and XNOR (1⊕2) out
• Four pots, one per step of the CV sequence
• A ring of four LEDs behind the panel's X
• 8 HP; Nonlinearcircuits publishes no current draw, depth or assembled price (it sells the DIY PCB and panel), and ModularGrid lists none