Exercise DET-05.06 – Camera flash of a smartphone

Prof. Dr. Thorsten Jungmann (last updated 2026-11-02)

Relates to DET-05 Codes, code converters and standard components

Task

The camera app of a smartphone offers the flash settings Off, On and Auto. With Off, the flash never fires. With On, it fires for each photo. With Auto, it fires only if the scene is dark. In simplified form, the inputs and the output are:

• $A$ and $B$: the setting, $A$ with the weight 2, with the code words 0:0 for Off, 0:1 for On and 1:0 for Auto; the code word 1:1 never occurs
• $C$: the scene is dark
• $Y$: the flash fires

Implement this function with standard components.

a) Set up the truth table of $Y$, and give the minterm list.

b) Implement $Y$ with a 3-to-8 decoder and an OR gate. Which outputs of the decoder lead to the OR gate?

c) Implement $Y$ with an 8-to-1 multiplexer. What do you connect to the data inputs $I_0$ to $I_7$?

d) Implement $Y$ with a 4-to-1 multiplexer, with $A$ and $B$ at the select inputs. What do you connect to the data inputs $I_0$ to $I_3$? Draw the circuit according to the standard of the International Electrotechnical Commission (IEC), IEC 60617.

e) Why does none of the three solutions need a minimization?

f) Suppose a software update gives the code word 1:1 a meaning: a fourth setting, with which the flash fires only if the scene is not dark. What do you have to change in each of the three solutions?

Solution

a) $Y$ is 1 in rows 2, 3 and 5, an X in rows 6 and 7, 0 in the others; $Y(A,B,C)=\sum m(2,3,5)+d(6,7)$
b) the outputs 2, 3 and 5; the outputs 6 and 7 stay unconnected
c) $I_0$ to $I_7$: 0, 0, 1, 1, 0, 1, and any fixed value at $I_6$ and $I_7$, for example 0
d) $I_0=0$, $I_1=1$, $I_2=C$, and any fixed value at $I_3$, for example 0
e) Each solution takes the truth table as it is: the decoder delivers all minterms, the multiplexers select the values of the table
f) decoder: lead output 6 to the OR gate as well; 8-to-1 multiplexer: $I_6=1$ and $I_7=0$; 4-to-1 multiplexer: $I_3=\overline{C}$, with an inverter

▶️ Full worked solution