Technical note

Shutter control circuit for photography of single oscilloscope sweeps*

Keywords--Camera, Instrumentation

Introduction

SINOLE-FRAME photography of oscilloscope sweeps requires that the camera shutter be opened before the sweep starts, and closed after it ends. These syn- chronising operations are usually carried out manually, or semi-automatically using the facilities built into physiological recording cameras. Fully automatic control, as outlined below, has obvious advantages including a significant increase in reliability.

Electronic circuit

The circuit principle is shown as a block diagram in Fig. 1. Cycle period for the system is determined by a clock-pulse generator. Each clock pulse is 'stretched' and applied to a control gate which opens the camera

shutter. The sweep trigger pulse is delayed by an appro- priate interval (50-100 ms) to allow time for the shutter fully to open. The control gate is then held open for the entire sweep, irrespective of its duration, by means of the unblanking waveform derived from the oscilloscope's time base.

Implementation of the circuit principle can take various forms, depending on the availability of pulse generators etc. The timing element in Fig. I could most simply be a monostable flip-flop with delayed pulse output. Fig. 2 illustrates a convenient design for the control gate. The first clock pulse to arrive following closure of Sw energises the relay, which remains latched during the high state of the unhlanking waveform. The shutter operation mode is set to BULB SO that the open time is gated by the second pair of relay contacts.

L~ stimulators

clock timing element

I control gate

I . . . .

_~r--L_

t unblanking pulse

.] '~-- --~.

�9

oscilloscope

b shutter

Fig. 1 Block diagram of shutter-control method. The timing element provides delayed and stretched clock-pulse outputs (50-100 ms in each case). Relevant timing waveforms are indicated for one cycle, the sweep time (variable) being indicated by broken lines

* F#st received 28th August and in final form 30th August 1973

126 Medical and Biological Engineering January 1975

o clock pulse in

Tr 1

B'I. Fig. 2 Schematic of control gate. suitable for use with positive control pulses

Tr 2

"1 I I ' ' 0 I unblanking pulse in

camera > external control

Versatility of control can be increased by the inclusion of counting circuitry to permit superimposition of n sweeps, or photography of every nth sweep. However, even the single-sweep unit provides a large measure of convenience in operation, especially when physiological events are to be recorded at widely differing sweep speeds.

Acknowledgments--This work was supported by the Australian Research Grants Committee and the National Heart Foundation of Australia.

R. D. PURVES Department of Zoology

University of Melbourne Parkville, Vie,, Australia

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