A handy clock generator that you can programme with one finger
Hands up all those who have trouble getting their drum machine or sequencer to produce rhythms at the right tempo. It is after all quite a lot to expect of a single turn rotary control to resolve tempos with the precision our critical ears demand.
Wouldn't it be great if you could persuade your machine to copy any tempo that you tapped out with your fingers? Well, now you can, using E&MM's TAP-TEMPO.
Tap-Tempo is a completely self contained clock generator utilising CMOS ICs to provide long life from an internal battery. To set a tempo, all you have to do is tap the clock switch twice at the desired rate, after which Tap-Tempo will produce a constant stream of precisely timed pulses at that same rate. The pulse stream can be stopped at any time by tapping the switch once.
For fast tempos, a X2 switch is included which, when the X2 mode is selected, causes pulses at twice the tap rate to be produced.
An LED monitors the pulse rate and also indicates that the unit is functioning correctly. This is especially useful if any triggering problems are encountered since it gives the user confidence in the unit. The battery is switched on when a jack plug is inserted into the output socket, eliminating the expense of a switch, and reducing the possibility of leaving the unit switched on when not in use.
When you are not using Tap-Tempo as a clock generator, you could record its pulses on your multi-track for click tracks. You could even plug in a crystal earpiece for use as a personal metronome.
The full circuit diagram for the Tap-Tempo is shown in Figure 1. IC3a, b&c form a 500Hz clock which uses three gate elements rather than the more common two, for improved stability. Tap Tempo has two modes: write mode when flip-flop IC1a is set, and read mode when IC1a is reset. When the unit is switched on, C2 and R4 set IC2a, which resets IC1a via C5, causing the read mode to be selected. When the tap switch SW1 is first closed, IC1a becomes set, and the write mode is selected. IC4 and 5 comprise a four decade BCD counter, which now counts up at a rate determined by the 500Hz clock.
When SW1 is tapped a second time, IC1a is knocked back into the read mode, and the accumulated count in IC4 and 5 is latched into the 8 bit register formed by IC6 and 7. The BCD counter is now preset with the contents of the register, from which it counts down to zero at the 500Hz clock rate. When zero is reached, IC5 pin 7 goes low, causing IC2b to become reset on the next clock cycle. IC2b will remain set for one clock cycle, during which time IC4 and 5 are preset with the contents of the register again. This process repeats continuously while the unit is in the read mode. The stored count, and thus the count down time is proportional to the time between taps. The components associated with IC1a ensure that all switch bounce problems are eliminated.
The preset cycle pulses on the outputs of IC2b are used to provide the Tap-Tempo's trigger output. By linking R10 to pin P, positive going pulses are available, whereas linking R10 to pin N produces an output with a positive voltage pulsing to 0v. Each pulse is indicated by LED D1 which is driven for a very short time at a reasonably low current to retain the low current consumption feature that the CMOS circuitry naturally provides.
During normal operation, IC1b divides the main clock frequency by two, so that the BCD counter is in fact clocked at 250Hz. However, when the X2 switch is closed, the read mode disables IC1b so the full 500Hz clock is presented to the BCD counter by IC3d. In the write mode, the dividing function is enabled, resulting in a 250Hz count clock. The consequence of this is that on the first tap, the decade counters count up at 250Hz, but after the second tap the counters count down at 500Hz, so the output pulse rate becomes twice that of the input taps.
If only one tap is received, then IC1a will still be in the write mode when the BCD counter reaches its maximum count after about one second, at which point pin 7 of IC5 goes low. On the next positive clock edge, IC2b will become reset, which clocks IC2a into its set state, causing IC1a to be knocked back into the read mode via C5. IC2b is frozen in its set state, so no output pulses are produced. This frozen condition is terminated on receipt of the next tap. The freeze condition also exists at power-up so that the unit will not produce any pulses until tapped.
Construction is made very quick and simple by the use of a single PCB to house all the components, including socket and switches. Tap-Tempo is not, however, recommended as a first project for the total newcomer to electronics since the tight and quite complex PCB pattern requires the constructor to be quite proficient with a soldering iron. Use a light soldering iron with a small tip, and fine gauge solder.
Referring to the component overlay Figure 2, populate the PCB as follows: Insert and solder the 18 wire links. Use insulated solid wire for all except the shortest links, which can be bare wire. Next insert and solder the 5 veropins and the resistors. Do not place R10 at this stage.
Proceed with the capacitors and IC sockets, but don't insert the IC's themselves yet. Now push the toggle switch SW2 firmly down into its position on the PCB and, after ensuring that it is perpendicular to the PCB, solder it in place. Glue a piece of 1/16" PCB off-cut to the standoff pips on the under-side of the push switch SW1, to act as a standoff spacer. Now apply glue the other side of the spacer PCB and push it and the switch firmly into place on the main PCB. When the glue is set (not long if you use super-glue!), solder the switch pins generously. Next fit the LED, the longest lead being the Anode which goes into the hole marked 'A'. Before soldering, adjust its height so that the tip of the LED dome is 15mm from the surface of the PCB. Now solder the battery clip wires to the appropriate terminals on the PCB, as shown in Figure 2. Note that the jack socket, JK1 is not mounted on the PCB at this stage but to ease assembly later, make sure that its pins are formed correctly so that it drops easily into its PCB position.
It's now time to check the PCB assembly very carefully with an eyeglass (or magnifying glass) on the track side to detect any dry joints or bridged tracks. Don't skimp the checking since a few seconds spent now can save hours of fault finding later.
If all is well, the IC's can be loaded into their sockets. Since the IC's are of the CMOS type, they are sensitive to static discharges, so keep them in their conductive packing until you load them, and discharge your body to earth before handling them. Also, of course, make sure you get them polarised correctly in their sockets.
Now decide what type of output you want. If you need positive going pulses, connect R10 between pins T and P. For an output normally positive pulsing to 0v, use pins T and N. If you want to be able to change the output polarity, the 3 relevant veropins can be inserted from the component side of the PCB so that R10 can be positioned on the track side.
To allow testing of the PCB assembly, temporarily link together the two JK1 terminal holes near the edge of the PCB. Now snap on a fresh PP3 battery and, with SW2 in the normal position (towards JK1), watch the LED. Initially, it should be extinguished. Tap SW1 twice in rapid succession. The LED should now flash at the same rate as the taps. Try this at different tap rates, noting that when the time between taps exceeds approximately one second, the LED no longer flashes. A single tap should stop the pulsing anyway. Now switch SW2 to the X2 position. The LED should now flash at twice the rate of the taps. If all is well disconnect the battery and remove the temporary link, making sure that the JK1 position holes are not obstructed.
After carefully preparing the case as shown in Figure 3, mount JK1 in the side hole with the terminals facing the open back of the case. While holding a 6BA screw in position from the front, slip on the 6BA spacer and feed the PCB assembly into the case so that the screw enters the PCB hole, and SW2 enters the appropriate case hole. Manoeuvre the PCB to locate the JK1 terminals in the PCB holes, push firmly home and solder. If the push switch doesn't align correctly with the case cut-out, then slacken the nut on the jack socket, and melt the toggle switch joints whilst easing the PCB in the required direction. Now secure the jack nut, the switch nut, and the nut on the 6BA screw.
After making sure that the unit still operates correctly, a piece of foam glued to the track side of the PCB against the 6BA screw will keep the battery in place when the lid is screwed on. Use super glue or self adhesive foam.
When you first plug in, Tap-Tempo will not produce any pulses, waiting in readiness for its first pair of taps. When you want your rhythm to commence, tap the push switch twice at the required tempo, remembering that the output pulse stream will actually start on receipt of the second tap. If you are more comfortable with a 1-2-3-4 start, then this can be done by tapping the 1-2-3 on the case of the unit followed by 4-5 on the push switch. The rhythm will then start on the beat following -4. To stop the rhythm, tap the switch once immediately after the last beat. The tempo can be changed at any time during a sequence by tapping twice, but remember that the first tap must be just after a beat, otherwise a pulse will be lost.
When the required tempo is too fast to be easily tapped in, or if it is more convenient to do so anyway, the X2 mode can be switched in so that the unit will pulse at twice the tap rate.
In most cases you should find that your equipment will trigger reliably on positive pulses since this seems to have been accepted quite widely as a standard. The 0v pulsing output will cater for most of the remaining requirements; any other problem requirements being solved using the E&MM Universal Trigger Interface.
Your Tap-Tempo will prove very useful for producing click tracks either by recording rough sequences, or by directly recording pulses, in which case the output should be shunted by a 470 Ohm resistor to 0v to attenuate the output voltage.
To use Tap-Tempo as a personal metronome, just plug a high impedance crystal earpiece directly into the output jack socket.
No more endless hit and miss knob twiddling to get your tempo just right. Plugin Tap-Tempo for precisely timed pulses, instantly on tap.
Feature by Paul Williams
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