Shares in security and surveillance systems firm Petards tumbled after it revealed a decline in half year profit and said results for the second half are expected to be weaker still.Pre-tax profit fell to £115,000 the six months to 30 June 2010 compared to £242,000 before. Revenues fell to £5.3m during the period from £6.6m before.Commenting on the current outlook, chairman Tim Wightman said, "We expect results for the second half to be slightly weaker than in the first half. Although revenue should be higher, margins are expected to be lower.""However, we do anticipate a significant order intake in the balance of the year which will provide a good base from which to build for 2011."Net debt increased to £1.9m from £1.8m in 2009.