Urgent help in bit_string data type

Hi Gurus,
 
   I have a table with datatype as bitstrings
 
 
create table test_a (b bit(3));
 
insert into test_a values ('111');

This will convert a bit to a number
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select to_number((substring(b,1,1)::int),9)+3 from test_a;

How will I convert a Number to a bit
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insert into test_a values (to_char(1,'9'));
 
ERROR:  column "b" is of type bit but expression is of type text
HINT:  You will need to rewrite or cast the expression.
 
Please help me
 
regards
skarthi